MMR Exploit
Funny how people are completely convinced that since World of Warcraft has been around for 7 years, that there isn't anything left to exploit and all of the holes have been plugged. :)
What's even scarier, though, are the people who believe that "hacks" and "duping" have never happened in WOW. I see these arguments on forums all the time.. and it's very depressing each and very time. (Duping is still a common problem in WOW, and it's going to happen in Diablo 3 too.)
Anyways.. apparently, MMR exploits in World of Warcraft are still around. I don't really follow WOW anymore and I just learned about this by accident. I thought it was worth mentioning..
In fact, I can't remember a time when there HASN'T been a Match Making exploit available in WOW. These types of exploits have been around since Arenas were first launched. More details here on Reddit, and here's a link to the exploit. Users are trying to get as many people to know about the exploit as possible, so that Blizzard will do something about it.
The GCD hack was always my favorite though.