Color schemes is a nice website for choosing colors that go together well. Read the About text, it contains some helpful hints.

Googling for "color scheme" chooser brings up other sites, and some of them contain more ideas, or code.

(Update: Ah, I remember now why I liked that first site enough to remmeber it. It lets you see what people with abnormal color vision -- of different sorts! -- see when they look at the combinations you choose.

Totally off topic, but I have to recommend "The Case of the Colorblind Painter" now to everyone. It's from the book An Anthropologist on Mars by Oliver Sacks.)