Good techie, but a little shaky on design and color choices? Need some help in working out styles for your skin, or are you a user who wants to use a custom style, but you're not sure how to work out a good color scheme?
Click on the "Base RGB" and enter a hex code for a color you like. Then tinker around with the settings to generate a range of lighter/darker, but still coordinated colors, complementary colors, and so on. A fabulous resource.
Stumbling onto this launched me into a cascade of skin rewrites (at the moment, I'm up to my "what's-it" in them), now that I have a way to generate things like beveled or hover borders that coordinate properly with the boxes to which they're applied.
Maybe also worth mentioning is colour legibility, several articles on the web. I've seen many sites were it is difficult to read text/links because of a low contrast between the foreground and background colours. It can be especially difficult for viewers with colour blindness.