Reports are suggesting that it's a beta version of the currently banned Manhunt 2 (on PS2) that has found its way onto the worldwide web.
Details are still coming in but it appears that a team of anonymous hackers has got access to the game and uploaded the software to the web, making it available for download on various sites.
Obviously this is very illegal and not something you should do. We'd also imagine Rockstar is trying to shut it down as you read this. It may have already done so.
We contacted Rockstar who was aware of the situation but offered no comment.
Stay tuned for any updates.