ISAPI_Rewrite
ISAPI_Rewrite is a powerful URL manipulation engine based on the regular expressions. It acts mostly like Apache's mod_Rewrite. It can be used to allow dynamic pages to be indexed by search engines, hide an internal structure of your web site, to correct mistyped URLs or strengthen server security. It supports variable rules and conditions to create really flexible URL manipulations, and you can find a lots of examples in product documentation.