I sat for hours in Design 101 trying to get a even coats of cadmium red, cadmium yellow medium hue, and pthalo blue acrylic and their various combinations in perfect circles to learn the basics of complementary colors and other ins and outs of color theory.  Now several years later, it all comes back to haunt me with Adobe's Kuler.  I've seen several for-pay downloadable apps to do this, but Kuler is free and web-based (Flash).  Simply pick from a library of pallets or generate your own using a variety of color relationships.  Use with Firebug for quickly knocking out CSS color schemes.