Not many IMO. Personally I don't like to see the coloured ones on as they simply don't blend with the rest of the white case(unless fitted with matching skins which isn't too bad I suppose).
There are a couple I like, mainly the launch team one which I think Batch from these forums has, that ones very nice.
Also depends alot on the position of your console, if you lay it horizontal you have more options than if you stand it vertical as I do, most seem to be designed for the Hori's out there.
The PGR3 one is nice(again, doesn't match the console though) but quite pricey, although I think fakes have started doing the rounds now which look very close & are way cheaper.
Hopefully some nicer ones will start to appear. I like the looks of the Dead Rising one, although I've only seen a mock-up picture.