Is there any kind of equivalent of Game Center or Openfeint for Mac OS X games?

The only one I know of is Steam, but it is more geared to hard-core gamers, not casual games. I'm working on a Mac OS X game that I will then port to iOS. So I'd like to have leaderboards and achievements that work cross-platform.

Is there anything out there that does this?

Thanks.

Mark

Unfortunately I do not think this type of system really exists for Mac OS X. Most people implement it themselves with their own server (as far as I know). Someone else may come along and correct me, but unfortunately I don't think it exists. Achievements have not been a very big thing with PC gaming in general before Steam I think.
As for Steam, I think on the Mac side it is a bit more for casual gaming (as mac gaming overall is more casual), so you could still give that a try. Or wait/hope for Apple to bring GameCenter to Mac App Store apps as well.
Alex