Many generous people have created and shared web-based and desktop color utilities. I have used a number of these tools for web design and graphic imaging and found a couple I just can't live without.
The color tool I depend on most is ColorCache from Technology Lighthouse. ColorCache is a color picker, color schemer, scheme previewer, palette manager and more. Currently in version 3.x, it is free to download and use for 15 days, after which paid registration (worthwhile, IMHO) is required.
My second tool of choice is a brilliant Firefox extension, ColorZilla color picker, DOM inspector and all-around groovin' color gizmo. I love this (along with WebDev for FF) for working on MT templates and styles.
Here are some useful web-based favorites to check out:
4096 Color Wheel Version 2.1
Color Palette Creator
Color Blender
Colours on the Web - Color Theory and Color Matching
ColorMixers
Colorblind Filter and Color Lab - excellent tools for accessibility to colorblind users
