ISAPI_Rewrite is a powerful URL manipulation engine based on regular expressions. It acts mostly like Apache's mod_Rewrite, but is designed specifically for Microsoft's Internet Information Server (IIS). ISAPI_Rewrite is an ISAPI filter written in pure C/C++ so it is extremely fast. ISAPI_Rewrite gives you the freedom to go beyond the standard URL schemes and develop your own scheme.
What you can do with ISAPI_Rewrite:
Optimize your dynamic content like forums or e-stores to be indexed by a popular search engines.
Block hot linking of your data files by other sites.
Develop a custom authorization scheme and manage access to the static files using custom scripts and database.
Proxy content of one site into directory on another site.
Make your Intranet servers to be accessible from the Internet by using only one Internet server with a very flexible permissions and security options.
Create dynamic host-header based sites using a single
physical site.
Create virtual directory structure of the site hiding physical files and extensions. This also helps moving from one technology to another.
Return a browser-dependent content even for static files.
And many other problems could be solved with the power of the regular expression engine built into the ISAPI_Rewrite.