So you are looking for some new colours for your latest design?
The first place you want to look is the Color Scheme Generator which will give you a palette of complimentary colours, as well as testing them for various colour-blind effects. Think of the Color Scheme Generator as doing for colours what Typetester does for fonts.
ColorBlender is perhaps even more useful although it is a matter of personal taste. Blender does much the same as Color Scheme Generator and can also output a colour table for Photoshop, as well as Pantone matching and more. The slider interface is very nifty and smooth.
Adobe has just launched Kuler, a way of sharing colour palettes for importing in to Photoshop, Illustrator, InDesign. With this you can quickly see, download and share palettes that are known to work well together.
Then there is ColorWhore, a simple site for sharing hex codes for colours, which is a bit superfluous after the others but still a simple and nice visual way of comparing colour bands.