Whereas StrangeBanana just tends to make random hideous color mash-ups,
Colorblender seems to do a great job of choosing a color palette. You pick a colour, and it automatically selects complementary and highlight colours. Works right in your browser, great use of sliders, and lets you download your new palette in a variety of formats.
Now if only there was a similar tool for screen design and layout...
(Via Mitch Denny)