What service to host global high scores and achievements?

[Stoney]

I have used OpenFeint as a developer and I found it really easy and confortable to work with. It has Game Center integration built-in for iOS, so you don't have to worry about implementing two services in your application. OpenFeint is something I will use in future mobile games.

As for user-side I have tried Steam as well and I think it's great. As for mobile services I have used Scoreloop, GameCenter and OpenFeint where I found OpenFeint to be the best experience with its pretty UI and more features than GameCenter.

[WILL]

Apparently Microsoft is coming out with a Game Center of their own, but compared to Steam it's not as good as you have that whole gaming community buying games through Steam. (more attraction than one that can't)

I think you'll want to try the bigger names with bigger communities for whatever platforms you're porting to.