I've been a big fan of Adobe's Kuler web site for a while now.
If you're not familiar with the site, it's a Flash based web site that lets you quickly browse through color schemes or make your own and quickly see how they'd fit together.
I always advocated taking a screen shot then taking note of the RGB values via an imaging tool.
Thankfully, Jonas Follesø, an RD from Norway, created an add-in for Expression Blend and Expression Design that links to the Kuler website via the site's REST services.
The project is available on CodePlex, so you can see how to create add-ins for Blend and Design.
Up until now, I hadn't thought about add-in support in Expression products.
Now, I have all kinds of ideas. :)
[found via Andrew Duthie]