StarForce
StarForce installs a driver in ring 0. Major drawback is if a game was protected by a SF Pro 3 system, it might not work or even reboot the computer if run on a 64bit OS. Just updating the SF protection is also not sufficient, the games developer/publisher would also have to release an updated version of the game.
Has been known to be distributed on demos, even demos distributed on Steam.
Has been known to cause system instability and hardware malfunctions.
For more information: http://en.wikipedia.org/wiki/StarForce