The standard ones: wallhack, aimbot, triggerbot, radar. In nearly 8 years of playing popular online games it would have been impossible to not find at least one of every type of hack.
My favourite is the CS(S) speedhack + aimbot combination. Seeing someone run through the entire level in a few seconds with an M249, never missing a headshot is awesome to watch but horrible to play against.
EDIT: I nearly forgot the Alien vs Predator 2 hacking. Someone discovered that with Tsearch, you could find the address for the smartgun's auto-aim and freeze it.(essentially making your own aimbot) It actually got patched years after most developers would have given up on the game. It's still possible, but a bit harder to do it, and that was only the start of AvP2 Tsearch 'hacking'.(last I found of it, was a video of an xenomorph killing others in spectator mode)