This is a great tool for generating color scheme ideas from a single color. You can generate complements, split-complements, triads, tetrads, analogous and monochromatic colors from your source color. The thing I really love about it is that has a button that will give you a random color to generate a scheme from, this is really useful for brainstorming color ideas.

I used this tool to come up with the colors on this theme (as of the writing) based on the triad colors of the shade of blue. This is one of the many applications that I enjoy that you can't find anywhere except on Linux.

Project Homepage
http://home.gna.org/colorscheme