Many times when you're designing a website (or planning on painting your home!) it's helpful to have a bunch of color samples or "swatches" which you can mix and match to experiment with different color combinations.
There are several websites on-line which allow you to do this. I think my favorite one is Kuler from Adobe (Flash-based):
Click on the "Create" button and try dragging the knobs around the color wheel to create new color combinations or "palettes".
You can also click on "Select A Rule" to change the method used for picking the set of colors in your palette (ie, it changes the way the knobs in the color wheel interact with each other) - using options such as Analagous, Complementary, or Monochromatic.
This is a very helpful interactive website which is fun to use and which can help anyone learn more about the art and science of color.