beaTunes MatchLists by Apple
Elysse.net describes Genius as:
kind of like a mixture between BeaTunes, Musicbrainz and LastFM, but less featured…
I would agree.
Watch out for beaTunes 2, which will give you even more control over automatic playlist creation. Additionally to the already established rules, it will let you specify filters to, say, create a MatchList only from a particular decade, or a particular genre, or both.
Now if only I could get Apple to pay royalties for the MatchList concept... ;-)