There are several schemes to choose from (complimentary, contrasting, triadic, etc) and knobs to adjust. You get 5 colors to look at next to each other and 18 along the same lines that go together. The input is in HSV/RGB and every output color comes in hex.
UPDATE: One thing that would be nice is to be able to drop a pre-existing color theme in and then tweak it with the color picker.