Zip Code Colorizer
Rowdy Labs has released a Zip Code Mapper to make this fast and easy.
While it is still very much under development, it is more than sufficient for many data display tasks that people have now.
I am doing development in a different place, so the above link should work at all times and be updated with the latest stable web site.
Currently scheduled updates include better error handling, a pretty print page, and a color picker control on the page.