AVG Decryption Tool For Bart is a free decrypter for recovering files locked by the Bart ransomware.
If you're unfamiliar with it, Bart is distributed via one of the largest malware-spreading botnets in the world and does not use encryption to lock your data like most but instead places your files inside a password-protected archive and deleting the originals, so you don't have access unless you pay up. Until now, of course.
AVG Decryption Tool For Bart is, for the most part, self-explanatory. However, AVG does have a tutorial available.