Nifty – if you’ve got an image in mind and want to see what the colour palette would be…
Image palette – a bit more clunky. You can either upload or use an url for the images. Not as attractive as Color thief, but you get the codes you need for re-use more easily. Still pretty accurate.
Pictaculous – Somewhat similar in style as Color thief, but you upload the images. It does not give as broad palette, and therefore it doesn’t feel as accurate to me. But the colours that come out does visually represent the image, although maybe not to the same depth that Color thief does.