Adobe Kuler is a website (and app) that helps you build a color scheme. (Kuler is the Creole word for color.) The iPhone app version is both fun and easy to use. You can quickly generate a coherent color scheme by taking a picture. The app draws from the colors in the photo. You can also tweak the auto-generated color scheme shades until it is perfect. You can upload your own color schemes to the Kuler website. You can also see what other people have made. Explore color schemes here.
Color schemes are useful when working in Adobe Photoshop, Illustrator, and InDesign (along with other programs). Both the app and website provide both RGB color values and hex (for websites). Kuler helps make sure your graphic designs and websites use color in a more coherent, complementary, and visually pleasing way.