There are plenty of utilities, sites, and applications that can help you pick a set of colors for your print or web project. But the one that's really knocked me out is ColorSchemer Studio for the Mac. It has the features you would expect: You can choose a starting color and let the application identify harmonious colors, browse variations of a single color, and create a library of your favorite palettes. You can even connect to an online database of user-created colors from within the application.
But what I found most impressive were the extra features. You can drop any image file into ColorSchemer and it will build a palette for you based on the colors in the image. It also features a layout previewer that allows you to drag and drop colors into a variety of templates for websites and print layout. There are also terrific accessibility features that allow you to check the readability of different text and color combination and even see what your color scheme would look like to people with different types of color blindness. By the way, although Colorschemer is usually priced at $50, for "a limited time," the Apple App Store is offering it for $20.
If you want to really dig in to working with color, you can also check out Nigel French's Photoshop for Designers: Color at lynda.com.