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.