Ok URL Rewriting is good.  I won't go into the reasons why but if your searching for it then you already know you want it. 

If you followed the standard progression you probably started with this MSDN article on URL Rewriting.  This implementation works fairly well in ASP.NET 1.x.  However, I've run into problems using this implementation in ASP.NET 2.0 with output caching.  For some reason it breaks output caching.

So what is the solution?

Well I'm glad you asked.  I've found a solution that works quite well with ASP.NET 2.0 and output caching.  The UrlRewritingNet.UrlRewrite assembly provides all the features of the original URL Rewriter mentioned above but also fixes the problems with output caching plus adds some interesting features like redirecting and programmatic access to the rewrite rules at runtime (say that ten times fast!). 

I'm currently using this on http://www.whatsyour20.com and I must say it works quite well.  I also like the fact that it is an active project and it even has a bug tracker system.  There is also a forum for this project.  That's always a good sign.

Enjoy!