The only one I can think of that not a lot of people know is:
If you have the Ray Gun, you let yourself get downed (Positioned in front of your entire team--On the Catwalk, for example)...this gives you 80 Rounds for the Ray Gun which you use to inflict Maximum Damage on in one of the Fatal Funnels. Blast away like a Mad Man...
Before you die, one of your Mates has to revive you...then you let yourself get downed and do it all over again. As long as you keep getting revived, then you can blast away forever. There is a video on it, but I don't have it to Post.
FYI...this isn't a glitch so much as it is a way to manipulate the ammo count when you have the Ray Gun.