well most PC games have tons of mods. Thats the one thing people like over xbox and ps2, is how simple to make, and how many mods there are for the games. Most of the time a 3rd person won't get banned, but if they are like really cheating (no damage, infinate ammo) they will probably get banned. I'm not a pc gamer, so don't quote me, but I do know some people that are and from what I heard mods are quite common.
Well a lot of times mods are really good when they are put time into. Like some BF2 mods and what not completely kinda off change the game into another. Really sweet when that is true.