Nuclear showdown is a gameplay mod for Duke Nukem that adds many features such as new weapons, enemies, effects to the default game as well as usermaps - it is built so you can play your favourite usermaps with, even ones that use custom art. You can also play through Duke it out in DC if you have it.
For a full feature list, check the bottom of the manual. Otherwise what follows is a catagorised list of the new weapons, enemies and features.
Enemies
Alongside the original enemies, you may encounter some of these in usermaps (and you will definitly see them in the 'Fallout Tour' episode)
Kamikaze Liztroop

Nicknamed the 'RTFM' troopers (people who have not read this Top Secret Dossier will easily succumb to their deadly trap) When you see these guys reach for their wrist panels, you would be forgiven for thinking that they're merely just trying to teleport around like you've seen so many times before. Of course, what isn't so forgiving is the fact that these guys pack some pretty severe explosives in said device, and that they plan to take you with them (as well as any of their allies in the surrounding area)
Liztroop Major

Liztroop captains who have been promoted, the Major is tougher (Thanks to his dark red armour) and fires a more powerful laser at you. Take care of these guys quickly.
Pistol Trooper

Althouh most Liztroops prefer the standard issue laser, some of them suit up in dark blue armour and wield a pistol much like your's. Whilst in solitude they are nothing more than a nuisance like their green clad brothers. However in numbers the number of bullets flying soon rack up.
Freezer Trooper

The Freezer troops wield a small Freezegun, and like the Shrinker Troops will shoot through their ranks just to reach you. The yellow armour they wear is slightly more durable than the other troopers.
Lizard Flamer

The Lizard Flamer has a flamethrower attached to his arm instead of the standard issue chaingun. Harmless at a distance, but deadly up close due to both the heat of the flames and the obvious visibility problems of having flame shot into your face.
Bare Octabrain

Ever wonder what an octabrain looks like underneath all that mass? At some point it resembled this thing. Not having developed the huge brain neccesary to channel psychic attacks, these guys resort to spitting acid loogies at you. Still not very pleasent.
Vacation PigCop

Remember how pissed you were when these alien bastards screwed up your vacation? Time to give these chilled out Pig Cops a taste of their own medicine.
Pig Special Forces

The Pig Special Forces are mutated version of elite EDF troopers. Clad in heavy dark body armour with night vision goggles, they can take a hell of a beating and will give some back to you as well. Watch out for ceiling windows as well, as these bastards have been now to rappel down them on ropes to give Duke a nasty suprise.
New Items and Equipment
Duke will have a wide array of new equipment alongside his old mainstaples to help him in his future battles. If Duke runs over ammunition for a weapnon that he already has a load of, the remainder will stay in the box for him to pick up again at a later time, preventing him fromw asting valuable ammunition. Normal pistols can also be duel wielded - if you locate another, you can use both of them at the same time.
The new inventory items can be viewed and access by binding a key to 'New inventory' and then pressing it ingame. Icons of all the new inventory items will show up, and you can select one by using the standard inventory left and right keys (the normal inventory is disabled whilst you are viewing the new items) - pressiny key will use/activate the new items when they're selected.
Spacesuit

The Spacesuit, not suprisingly, protects Duke from the negative effects of standing in outer space (ie; dying a horrible death) It automatically equips itself when Duke is exposed to Vacuum, however whilst wearing the bulky suit Duke can't use his weapons. They probably wouldn't work in space anyway.
JumpBoots

The Jumpboots, when activated, boost Duke's jump by an incredible amount. They also act as shock absorbers and will protect Duke from any fall as long as he doesn't reach terminal velocity. Each set has a charge of 3 jumps.
Toolbox

Duke's not just a handsome guy - years of tinkering with his favourite vehicles has left him highly skilled with Toolkits (as well as high powered weaponry of course) With one of these, Duke can repair faulty equipment and buttons. The Toolkit will automatically be selected if you're near something that needs fixing - just press the use inventory key to fix it.
Backpack

The backpack doubles Duke's carrying capacity, allowing him to dish out more damage without having to worry about ammo so much.
Thermonuclear Health

The Thermonuclear health is the Atomic health's big brother. Not only does it add 100 health over you maximum limit, it can also add over your maximum health with atomics as well. Meaning if you find one of these babies, you can have over 200 health.
MP5K machine pistol

The MP5K is the automatic counterpart of Duke's standard pistol. It fires faster, and has a larger magazine of 30 rounds. The best part is if you find another one of these, you can duel wield 2 just like the pistols. However, it is more innaccurate than the pistols meaning that for distant targets switching back would be a wise move.
Lead Cannon
Although the standard Shotgun is nice, sometimes you just want to hurl 4 simultaneous blasts at the exact same time towards a stubborn foe. The Lead Cannon answers this call. Clumsy, with low accuracy and the inability to do anything except hold and fire 4 shells, the Leadgun still is a valuable tool when the going gets tough and you have shells coming out of your ears.
M60 LMG
The M60 LMG is a chain-fed machine gun more commonly used in vietnam, although this upgraded model is similar to the one Duke wielded in the GLOPP incident several years back. Although it requires reloading unlike the Ripper, it is much more accurate - perfect for use when you need to snipe off a solitary enemy in the distance.
Desert Eagle
Duke loves his Pistol, but when the going gets tough he prefers to turn to his beloved .50 calibre Hand cannon. This baby can drop a Pig Cop in a single shot, and is accurate to the punch as well. The only problem being that ammunition is hard to come by, so savour this portable Howitzer for when you REALLY need it.
Proximity Dynamite
The Proximity mine does as the label says - chuck it and it'll pick up any enemies (Dead or Alive) and explode with a powerful blast. Make sure you're well clear as it hurts like a bitch.
Plasma Gun
The Plasma gun is one of Duke's most deadliest weapons. Built with a charging feature, just holding down the trigger charges up a burst - letting go unleashes the gun's beutiful creation into the big wide world, ready for it to slam headfirst into whatever nasty Duke is facing at that current moment.
Features
What follows here is a list of all the new stuff in Nuclear showdown. This list isn't comprehensive, as I may forgot or exlude things for various reasons.
-The Pistol and Shotgun can now be reloaded at will, and have a counter display the current magazine count.
-Boss enemies now make a different stomping noise when walking, and if you're too close the ground shakes. The ground also shakes when they fall down and die. Also mini-boss versions for the other 3 bosses now work.
-Footstep sounds, and landing sounds added.
-Enemies now suffer damage from things like Lava and slime, and some can also drown underwater as well (They can hold their breathe as long as DUke can, but since they can't surface they're doomed anyways)
-You can now play Duke it out in DC via Nuclear showdown.
-Enemies now make a unique sound when they fall over compared to the generic thud noise.
-Liztroops now act differently when they try and play dead.
-Being hit by a flying drone sends Duke sprawling backwards, like being hit by a fast exploding metal object would normally do.
-PigCops (All kinds) can ram the player with their weapon if they get too close.
-Babes who are captured by the aliens can now be saved. Just press space and she'll be teleported to safety, leaving the remains of the alien slime, and if you're lucky one of the new weapons for your troubles.
-Pigcops and Lizman can be headshotted.
-Pistols, Shotguns and rifles spawn different kind of shells that bounce when fired. The Pistols also spawn an empty magazine when reloading
-Some new Duke quotes, used in various circumstances (Some for when you kill something, pick up a weapon, save a babe .etc)
-Emptying ammuntion or a large health will leave the box of the item behind.

-Scuba gear now has a fullscreen mask, and if you don't have scuba-gear you can see your remaining air in a counter to the upper right of the screen.
-Armour now absorbs all damage recieved from weapons, and is shown in the upper right corner when you have it. When hit with armour, your screen will tint slightly white instead of red to show that you have been hit but no damage has been done.
Bug Fixes
What follows here is a list of bugs or general nuisances that I've fixed in Nuclear showdown. This list isn't comprehensive, as I may forgot or exlude things for various reasons.
-Several objects that exploded but did no damage now do.
-Newbeasts are now resized automatically on level start-up rather than when you see them
-Several default sprites that were indestructible are now not (The captured female's green pod for example)
-Enemies that are made transparent in BUILD now retain that transparency when they die as opposed to become opaque again
-Spawned Liztroops now retain their spawner's palette, meaning that different liztroop types can now be spawned.
-Liztroop and Lizman gibs retain their owner's palette, meaning if you shoot a red liztroop than his gibs will be red as well.
-If a usermap maker makes lava a water sector, if he doesn't give it a red palette than the water splashes spawned in it will be blue. They will now be red.
-The computer tiles can now be broken by bullets if they're applied to floors or ceilings
-The computer screens can now be broken if placed as sprites, and they also randomly flicker like their wall counterparts
-Glass tiles, if given a spritepal of 3, can be broken if they're made into sprites
-Camera screens can be destroyed if given a spritepal of 3
-The previously unbreakable Monitor tile (Tile 638) can now be destroyed by bullets.
-Pig Cop tanks now leave debris behind when destroyed
-Pig cops no longer keep dropping Shotguns if they are frozen over and over again
Fallout Tour
Fallout Tour is a episode comprising of usermap levels. At the current moment it contains 6 maps, however this will hopefully increase in future maps. The maps featured, in order, are:
Plaza by Michael and Robert Caraballo

Trafic Jam by Olivier Boucher

Blind Alley by Alias Conrad Coldwood

Sewers and Strippers by Rob Wijkstra and Geoffrey

Lost City by Mariusz

Wet Dream by Rob Wijkstra

EDF Base by James Stanfield (Yours truly)

Credits
The following authors for their levels in Episode 6 Fallout Tour:
Michael and Robert Caraballo
Olivier Boucher
Alias Conrad Coldwood
Geoffrey
Rob Wijkstra
Mariusz
Ben Roffelsen
James Duncan
Eric Boltjes
Eddie Campbell
Mike Beaulieu
Nathan Orr
Scott Haslam
Bobby Portis
Robert Travis
Maarten Pinxten
If any of you object to your levels being in NS, please contact me by E-mail (Jstanf35@hotmail.com or Jstanf36@ntlworld.com) so I can remove your map. Thanks.
The readmes for their maps have been included
3DRealms for Duke Nukem 3D
TerminX for Eduke32 (And credits to Jonof and Plagman, as well as anybody who helped these guys)
Cory Whittle, for Immoral Conduct which was a huge inspiration
George W. Bernard, for Chimera, Last reaction and Water base (which I used some code and art from)
DeeperThought, for Code help
The Fly and Jonah Bishop for their Con supplement which I got some ideas and graphics from
The Survival team, for putting up with my lazy ass (seriously you guys are great)
Monolith, for the Dynamite Graphics and some sounds
Sunstorm, for the Pig Cop Vacation graphics
Ian Jones, for the MP5K graphics
Sang, for being the first to test both of the alphas releases so far!
and you, for downloading Nuclear showdown!
If you should be on this list and aren't, I apologise - please contact me so I can rectify this