For the longest time I used the Visibone Webmaster's Color Lab for the rare times when I had to come up with a color scheme for something webby. (I still do, really; it's pretty neat.)
Reading Kottke's remaindered links list the other day, though, I came across Pixy's color scheme chooser, which instead of only comparing colors together lets you choose a starting point (HSV) and then suggests colors to use in a monochromatic, contrasting or analogic scheme, and then lets you view how those colors will look if limited to web-safe colors (is there even such a thing anymore?), and with a bunch of different kinds of colorblindness. Way cool.