ISAPI_Rewrite from Helicon Tech. This is a URL rewriter for Microsoft IIS.
There are two versions still in use, with different syntax, so it is helpful to specify the version in the question. v3 is the latest, compatible with Apache mod_rewrite. v2 is the previous incompatible version. Both versions of ISAPI_Rewrite are really meant for IIS6 and earlier. For IIS7, Helicon Tech strongly recommends their Helicon Ape product, which contains the mod_rewrite as one module.