NSOutlineView, with additions and/or a custom cell class for display.
I've basically wrote one of these, and keep extending it to become more and more complete as time goes by. You can get basic functionality pretty easily (the background, selection style, and even the group label font is all supported in the Leopard APIs), but complex stuff like matching some of the functionality of iTunes' lists (auto-indenting of the outline view, badges for item counts or the like, etc) will require some extra code on your part.
If I clean up my code one of these days, I might make it public.