The ESPN PlayerRater rankings are decent - they are based on real stats/points and you can sort/filter many different ways.
CBS releases one every week per position, but it's ridiculously inaccurate. It's not based on any stats, rather just on the CBS experts' votes. Some players on some weeks are not even included, and you get the feeling that a lot of those rankings are based on reputation and not on this year's performance, and certainly not on recent play or streaks.
For example, ESPN ranks Scott Olsen and Roy Oswalt as the 32nd and 33rd rated SPs, respectively, based on this year's performance. CBS, meanwhile, has Oswalt at 9th and Olsen at 65th.
I don't know of any more free ones, so if anyone else does, please mention.