Bet on fantasy football for free!
It's done in a salary cap format, which means based on the 'fake-money' you're given, you buy players to fill up a roster. Never used this site until now, but footballoutsiders twitter account recommended it. Though your 1st entry is free of charge, you need to put money down if you're going to join other leagues. I put down 5$ via paypal, I'll update this next week if anyone's interested.