I've used this one before, it's a great little program!
I do find that I use Fireworks more often than anything else for determining and/or developing palettes though - but these tools come in handy when you need to create a quick family of colors or play around with unique combinations.
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Create a welcome packet for new clients with this guide and free templates!
(Available in PDF and Kindle versions)
This will be very useful in the new version of my site! In fact, you are a lifesaver as I am still deciding whether to use a black background with bright colours...