Built to demonstrate the capabilities of the latest versions of Flash and Flex, it's a web-based version of the extremely useful colour space explorer tools in Illustrator CS3 - with an added dose of community. Use the colour space explorer to choose a set of colours, then use rules to find complementary sets, or the appropriate colour triads, or even related shades to a base colour. It's an excellent tool for building colour maps that you can use in your web sites or desktop applications - you can take the RGB values of the colours you choose, or the hex.
What's even neater though, is the community, where people are sharing the colour sets and themes they've defined. A voting system lets you say which themes you liked the most - I'm particularly fond of Japanese Garden...
There are also RSS feeds of the most popular colour themes - so you can stay updated without having to visit the site every day - and OS X users get a Kuler dashboard widget.
An excellent tool for web and graphic designers.