Tom's Hardware has a very extensive and insightful write up on cheating in online games, running through some of the cheats that are being used in many of the popular online games at the moment (such as Counterstrike, Warcraft III, and Battlefield 1942).
"Cheating is now close to destroying entire gaming communities (Counter-Strike anyone?), and it's gotten to a point where developers are having to spend a large portion of their research and development time creating barriers against the individuals who program and utilize such cheats.....
Name any multiplayer-enabled game you own and it's almost certain that there has been some sort of cheat developed for it, whether it's a first-person shooter (FPS), a real-time strategy (RTS) or even a role-playing game (RPG). "
It's pretty interesting to see all the varied ways people use to get that added advantage. Read it over at Tom's Hardware. Slashdot has it covered.