TeslaCrypt ransomware has been used in massive malvertising attacks since its launch in March last year. The malware, which often targets PC gamers, locks up files until a ransom is paid, usually $500 in Bitcoin. Cybercriminals behind the malware have apparently shut down their operations and released a master key to the public that can unlock all encrypted files on PCs infected by the latest versions of TeslaCrypt. The master key is free, so victims can decrypt their files without paying a single penny.
Source: https://thehackernews.com/2016/05/decrypt-teslacrypt-ransomware.html