In David Brin's The Practice Effect, the main character has a weapon that basically looks like a pistol. However, to give it ammo you put any piece of metal you can find into the handle where the magazine would normally go. The pistol then shaves off pieces of the metal and propels them (using gas taken directly from the atmosphere, if I recall) at anything you point the trigger at. The beauty of the pistol is that it needs no propellant because it culls gas from the atmosphere and any metal that can fit in the handle is potential ammo. It was solar powered, too, so with air, sun, and any metal you have unlimited ammo.
Also, someone mentioned Saberhagen. In one of his Beserker series, the humans attacked the beserkers (robot ships bent on the destruction of all life in the universe) with what they called "C-plus" weapons. These were ships which carried one huge, massive "bullet"--the bullet itself was ship-sized. When the bullets were fired, they would skip in and out of hyperspace so that they couldn't be blown up. Right before the target they would exit hyperspace and strike the target at virtually light speed. Bye bye target. Think of a Titanic-sized solid mass striking a spaceship at that speed. The energy involved would be obliterating.