Lulz Security (aka LulzSec) have been responsible for some notorious hacks in the past 6 weeks that have made headlines, including the attack on the Sony PSN (Playstation Network).
Thousands upon thousands of Facebook, GMail and PayPal accounts have been hacked and worse made public. Gizmodo has set up a tool to allow you to see if your account(s) has been compromised. Check it out here.