pydoc. It helps you generate documentation as you are working the code and as a consequence it provides a great incentive to work on documentation from the start.
One of the nice features is a web front end to the documentation. If you are interested, have a look at our documentation. This is the documentation of the bleeding edge, so you may see things that have not even been committed to SVN. To get an updated to the latest version in SVN, use update_self from within the installer.