Cryptolocker
From Just Solve the File Format Problem
Cryptolocker is a rather infamous piece of malware that has infected many PCs. It encrypts the files on the system against the owner's will, then holds them for ransom; the malware distributor demands a payment (often in Bitcoin) in exchange for the key to unlock the system and get back one's files. However, a group of security firms have broken it, and have produced a free site that lets users decrypt their files without paying off the crooks.
