I'm a huge fan of Color Scheme Designer...
http://colorschemedesigner.com/
From the Lifehacker article...
You can generate single monochromatic,
complimentary, triad, tetrad,
analogic, and accented analogic color
palettes. You can simulate color-based
vision disorders to see how your
design colors will look—they even list
the percentage of people suffering
from the disorders. A preview function
populates a dummy web page with your
color scheme, which is a handy tool
for seeing how your selected colors
look together off the palette.
While the page-simulator is a really
great trick, the best feature of Color
Scheme Designer is the ability to
export your palette not just as a
Photoshop palette—a common limitation
of many web-based generators—but as
HTML+CSS, XML, TXT, and GPL (the
palette format for GIMP).
