That’s one cool web-service– say you need a set of ‘matching colours’ (for whatever fancy reason): Pick a base colour and let colorblender do the rest. (Doesn’t seem to work for me, though: Everytime I start with a colourful theme — say, for a website like this — I end up with a friendly set of grayscales. Uhm…)
Anyway, this site must have been one of the early birds when it comes to building something actually useful right within the web 2.0 hype.
Update: Here’s another one — Colorjack! A tad more complicated and loaded with features. Enjoy!