I'm working on (surprise) a web framework that entirely is dependent on mod_rewrite, however I'd like for it to be compatible with IIS ( if possible ). There's a slight slight chance one of the sites being built will have to be stored on an IIS box, I'm wondering if there's any rewrite module in the most popular versions of IIS I could rely on.
IIS7 is easy--use the URL rewrite module as Garethm points out.
IIS<7 is a bit trickier--no integrated pipeline to latch on to. Presuming you can require that persons hosting on IIS have dedicated hosting, or can at least get an ISAPI module installed, check out iconic's Isapi Rewrite Filter. It is free, open source and pretty much mimic's mod-rewrite.