I've found a couple of glitches in my time:
Crash one: Fight N. Brio, and get to the point where he's drunk the liquid. Get him down to his last life point, then stand on the falling ceiling tile. Wait there, and let him come up to you. He'll collide with you (and I can't remember, but I think you'll lose a mask), and then go back to the start and his health bar will disappear, but he'll keep charging you, and will rebound back every time he gets to the ceiling tile, getting faster and faster.
Tony Hawk's American Wasteland: Make a special manual. Save it to your memory card, and assign it to your Story skater. Turn off the game. Reload the game and make a special air trick in the create a trick editor (not in story mode). Turn the game off. Go to your browser menu, delete the special manual. Start the game. Your special air will now be in the slot where your special manual was in story. This has the upside of being a trick that works as a manual, and doesn't stop when you get launched into the air off a ramp.