Cooler Colour
There is this awesome super useful colour tool that I've been using for the past while. Kuler by Adobe.This tool allows you to create your own colour swatches and themes, and save them and view other people themes. In the create section you can create a base colour, with the option to change the relationship between the other four colours (Analogous, Monochromatic, Complementary.. etc) resulting in a visually appealing colour scheme that can be used in websites, print, or just to show off to other users of Kuler!
I like that you can input and view RGB, CMYK, HSV, LAB and HEX in the creation screen. And the very useful and user friendly colour wheel picker.
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However, I have not actually used the creation screen much, other than to play around with some colour choices a few times. My favourite feature is the ability to add the Kuler RSS feed to Illustrator, and import the swatches.
This is extremely useful when wanting to try out new colour schemes and immediately have the colours ready to be applied to my work.
Website: http://kuler.adobe.com