Pokerhost has frequent freerolls but I would read their cashout policy before spending too much time there. It basically states that if you don't deposit you will never be able to cash out your freeroll winnings. What I do is visit various sites at various times depending on who has the best game at the time.
Actually I think it might be Absolute Poker, they not only have frequent freerolls, but they have them in a variety of games and they arent as large as a lot of other websites freerolls