Reading the thread's title, my first though was the
machine gun in Cave Story. The idea is that, thanks to its recoil, you can use it as a kind of jet-pack, so suddenly you fly around levels thanks to your gun.
Second idea was
Another World's laser pistol. You tap to shoot a laser, you hold a little and release to get a static shield (that protects against a lasers for a while or for a number of hits), you hold a while and release to shoot a charged laser that can break a shield. So single gun, single button and suddenly you have some pretty intricate gun fights.
Reading the thread and seeing
Fear mentioned, I remembered the cool effect of the
nail gun in that game. It would literally nail ragdolls to walls. That was pretty impressive even if it had no impact (pun not intended!) on gameplay.
Thinking a bit more I am also a big fan of the
machine gun (?) in
Axelay. The idea is that when you press the fire button, you have two streams of bullets that fire behind your ship and slowly move toward the front of your ship like two hands on a clock that go in opposite directions. When you release the button, the gun still shoots, but the two hands rotate toward the back of the ship. This allows the player, with some nicely timed presses and releases to shoot in just about every direction he wants. Very nice friction on that gun.
To add a non-gun weapon....
Scrooge McDuck's cane in DuckTales 