I've dedicated one of my FO3 characters to collecting special weapons, and I'd like some help finding them.
Here's a list of special weapons that I've already found:
A3-21's Plasma Rifle
The Terrible Shotgun
Alien Blaster
Vampire's Edge
Ol' Painless
Firelance
Junder Plunket's brass knuckles

And that's about it.

http://fallout.wikia.com/wiki/Fallout_3_weapons

Look at the notes at the right of the graph and it will tell you whether its rare or not.