Adobe has a free web-based application out called Kuler.
In a nutshell, Kuler is a color picker that helps you design a color scheme of complimentary colors. It lets you start with a base color, and will generate a family of related colors based on some simple rules (whether you want a high contrast or low contrast color scheme, for example). Any of the automatically generated colors can be overridden as well, should you happen to want to.
Where Kuler really shines, however, is in the community around it. You can publish and share and browse other designers’ color schemes. You can rate the schemes you browse, and the best and most popular ones naturally rise to the top.
Color schemes are annotated with RGB, CMYK, LAB, HSV and and HEX/HTML color values, so you can punch ‘em right into your application of choice. You can also export your scheme as swatches compatible with all Adobe Creative Suite applications.
For people like me who have enough design sense to know that we design poorly, but not enough to design well, a good color scheme can mean the difference between an ugly garish design (this site) and wonderful attractive design.
Kuler is 100% free. If you do any design work at all, you owe it to yourself to go check it out.