Hacker group LulzSec has claimed responsibility for bringing down Minecraft's login-servers, EVE Online, League of Legends' login-servers and The Escapist Magazine's website yesterday. The group used DDoS-attacks against the various servers to bring them down, with at least CCP taking down their own servers when they noticed that something was going on.
At this point it doesn't seem like anything was actually hacked, or that the group had any motivations except doing it for fun. LulzSec has been linked to all kinds of hacker attacks lately, including the PSN hack, Bethesda and Nintendo.