If you are using ASP.NET 3.5 ASP.NET Routing could be a good choise for you.
MSDN page: msdn.microsoft.com/en-us/library/cc668201.aspx
Using it with ASP.NET MVC at ScottGu blog: weblogs.asp.net/scottgu/archive/2007/12/03/asp-net-mvc-framework-part-2-url-routing.aspx
Using it with ASP.NET 3.5: www.techbubbles.com/aspnet/aspnet-35-url-routing/
If your website runs under ASP.NET 2.0 Helicon ISAPI Rewrite could be a good choise for you. This is an IIS filter that redirects requests to your pages according to regex-based configuration file. They have a free version for one website.
Have a look at Helicon: www.isapirewrite.com
P.S. Sorry for not making links hyperlinks - "sorry, new users aren't allowed to add hyperlinks" :)