I've hacked in many games, I used to use the teleport hack in Everquest that would allow you teleport to any coordinate location in any zone, and had zone in's and outs on hot key, also allowing you to set up hot keys to use to pull mobs by teleporting to them aggroing them and then teleporting back to the group, before the days of "Aggro ranges" where people can now run away from mobs if they're scared without zoning out. This one was developed and published by a developer of Everquest, and was widely known about but unaddressed because it was published by a known developer in the program.
There are many hacks that can be applied, but often a "hack" is not a hack at all, and is just a mod, which are two different things. Illegal mods are entirely different than hacks, in a client side game you can create illegal mods to improve your game play, but it is near impossible to hack the game to benefit only yourself. In other words, if I want to fly my F2H and hack the game to give me unlimited HP and never miss, if I run into another F2H while I'm doing it, they will have the same strengths, and my hacking was pointless.