Uncharted 3: Drake's Deception has been a PS Plus freebie in the past, but today it's showing up on United States' PSN as free for all users. We've reached out but are yet to verify with Sony if this is correct or some admin mistake. So, just consider this your heads-up with a grain of salt until we find out.
Developer Naughty Dog did make the multiplayer side free-to-play just over a year ago, but single-player going free for all would be something else. E3 is just around the corner, so who knows what the Californian studio has up its sleeve?
Update: Ladies and gentlemen, Nathan Drake has left the building. Sony has removed Uncharted 3's single-player from the PlayStation Store, so we're guessing - just maybe - it was a goof all along.