DoubleTwist

Songbird

I've used both and I think Songbird is a little further along and more focused on you using your own library. DoubleTwist makes it really easy to use your existing iTunes library.

I'm not positive how "light" each of them is, but I've tried them both and liked Songbird a lot. I'd still be using it if I didn't have a Zune Pass for my ZuneHD.

Oh, and Bruno hates Songbird wink
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Matt