There's too many glitches I find to list...
The hole in Sonic Adventure: when you're making that right turn to the "docks" where the whale is, hold right you'll fall through the world.
Gears of War tables: You can make tables float in Gears of War, keep rolling at them or blow them up, you can get them to float around for a while, or if you get in a corner you can bounce on them. (if I find the vids of me doing it I'll upload them).
Fallout 3 through walls: when fighting enemies sometimes you can clip them through the wall to the next room, or fall through the floor, I've seen it happen 5 times I dunno how to do it but it happens.
Sonic 3: in ice cap zone when you start riding one of those sliding ice blocks hold duck, you'll vanish and the screen will fly left, right, left showing just the ground, then you'll appear bouncing on a spring.
Phantasy Star IV Over leveling: if you get past level 99 you'll actually start rapidly becoming weaker, losing moves, skills and stats VERY fast.
Team Fortress 2 abandoning 2Fort: I can't do it when I want to but I'm frequently flying far off the level in a server with no mods (besides increased people+insta spawn)