Excellent for use with Adobe Creative Suite 2, 3, 4.
Kuler is a web-based color tool from Adobe that has thousands of community-generated color themes from which you can search, but the best thing about Kuler is that you can easily make your own themes. Once you register with Kuler (it's very quick), you can begin to create your own color themes or palettes and store, view, and retrieve all your saved themes in your personal Mykuler space. There are two ways to create unique color themes: either by selecting a single color on the color wheel as your base color and building off of that, or by importing an image from Flickr or your hard disk and extracting harmonious combinations from the image.
If anyone is thinking about using a photograph as a source image for their logo, I came across this cool (even if slightly out-dated) tutorial on how to create a simplified vector image from a bitmap using live trace in Illustrator.