Color Oven is a great tool to have for all kinds of designers out there, especially Web designers. With it you can generate a color scheme by choosing an initial color and the scheme type (monochromatic, complimentary, analogous, or triad). Then you can save the color scheme as an html file so you can keep it for reference, print it, or even email it to someone across the globe. If you're a Web designer you can export the color scheme as a CSS file and import it to any website you're working on!
Mac OS X 10.4 or later
*Previously available here