World of Warcraft has an interesting feature: customizable UI. Downloadable mods like Cosmos, CTmod, GypsyMod, and Telo's AddOns let you add new UI elements to the screen like extra hotkeys, item databases, buff timers, etc. The WoW client is even scriptable in Lua, allowing some fairly complex things.
I've always been fascinated by the duality of client/server games. The primary user experience is the graphical client but the reality is the protocol underneath. It's like seeing the code beneath the Matrix. Back in the good ol' days of Netrek a popular hack was to show your opponent's position with more precision than intended, even when they were cloaked. The data was there in the client, so why not display it and gain an edge?
The security problems intrinsic to interface hacking have never been fully solved. At least modern games no longer trust the client. A big problem is macros like fishing bots; leave your character online with a script running and come back a day later to lots of ill-gotten wealth. You can even auction off the results for real money on eBay, except that Blizzard is trying to stop that.