Just a few links to work with color; you might know them all, or not, but if you have more, don’t hesitate to post them in the comments!

• Name that Color
• Color Palette Generator
• Adobe Kuler
• COLOURlovers
• Hex Color Picker for Mac OS X 10.4 “Tiger” (this is bundled with Leopard by default)

Finally, a classic – even if it has nothing to do with color: typography.com.

Enjoy!

Update, 2008-01-28: You can add Mondrianum to this list :)