Off the top of my head, some things that could be added. Perhaps some aren't as useful as others.
There’s http://mothereffinghsl.com/ for that.
i prefer hsl over rgb and would be happy to see hsl replace rgb on css3please. we could also link to mothereffinghsl for a richer UI
border-image.com is good for that. plus they are weird.
radial i ignored because they were hard. i think a present with a soft central gradient.
we also have a few outstanding issues already in the tracker i believe