Mod ReWrite
URL Rewriting is a very cool functionality that every webmaster should learn how to do. There's some really cool trickery that webmasters can do to make their website more productive and functional to the end users and developers.
Apache set the standards in URL Rewriting functionality. There's plenty of plug-in tools for IIS and other Webservers. However, Apache set the standard, and its very helpful to learn to use Apache's tool.
Download the Apache Mod Rewrite Guide to learn more about the power of HTTPD Rewrite functionality in Apache.