Part of the WellStyled.com web design workshop, the functionality of this color scheme generator is primarily aimed at web designers, but this is fascinating and fun for anyone who likes to immerse themselves in color and color relationships.
Click on the color wheel to choose a hue, choose a scheme (Contrast, Triad, Tetrad, etc) below that, and then click on the small rectangles in the color spaces to bring up a value/saturation picker. There are some other variations at the bottom of the main panel.
Any time you arrive at a scheme you like, click on “URL of this scheme” to bookmark the settings. There is even a dropdown menu for previewing how your scheme would look to people with varying types of colorblindness. For more info and instructions, see the Help page.