Here's some of my ideas:
Fists: Strong Right
- Knocks the enemy unconscious. It won't be able to do anything for a while. Takes more time to perform than a normal attack, and won't do any real damage.
Combat Knife: Throat Cut
- Fast to perform, and kills instantly if succeeds. Very low hit rate.
Chainsaw: Massacre
- Simply put, thrusts the chainsaw deep to the enemy's gut, dealing a ton of damage but taking rather a long time to perform. In addition, the chainsaw is lost - it'll be permanently stuck to the enemy's body, and can be retrieved from the corpse afterwards, although it'll take a more time to pick it up than anything else.
Pistol: Execution
- Quaranteed hit, double damage, takes more time to shoot. Can only be performed from close range.
Shotgun: Knock Back
- Pretty much what the first poster suggested: Knocks the enemy back a little.
Combat Shotgun: Durr...
Double Shotgun: Durr...
Chaingun: Bullet Rain
Shoots 15 bullets at once. Nigh impossible to hit the original target: Instead, the bullets spread up pretty much in a same fashion than a shotgun. Takes quite a more time to fire, though.
Rocket Launcher: Rocketeer
A combination of a single rocket, a rocket launcher, and large amounts of suicidal insanity: Fires the main character in a single direction, and continues going on until he hits something. The rocket only has a 50% chance of exploding afterwards, though, although both the main character and the target will take damage on impact. Convenient for a quick escape.
Plasma Rifle: Railgun
Like the first poster's suggestion: Goes through the enemy to hit the following one(s).
BFG 9000: BBQ 9000
Fires a single extremely powerful ray that will hit a single enemy only. However, this unfortunate hellspawn will be killed instantly (CD and JC are exceptions), after which the body will explode like a rocket, causing damage to everything too close as well. Takes a long time to fire, and spends 50 power cells.
LS: Purifying Light
Creates a field of energy that will remain there until the end of the game and deals moderate amounts of damage to any enemy that will pass through - like a pool of acid or lava, except that it's harmless to the player. Can only be performed once per level, and will take quite a while to do.
Well? Well?