0to255
Too much choice I think. "0to255 is a simple tool that helps web _designers_ find variations of any color." Designers already have offline tools to explore full color spectra.Tools that turn color theory into recommendation algorithms are much more useful. Illustrator introduced a "Color Guide" palette for this and it's great. Would be even better as a free online tool. Doesn't necessarily need to be user-contributed like Kuler.