The "C# WebServer" is an open source, web/application server that runs on both DotNet and NovellMono. It is packaged as a library and through a series of calls, you can set up various components such as an HttpListener, sessions, file serving, redirect rules, etc. It's very modular.

•  http://www.codeplex.com/webserver
•  Cobra discussion

You can get the latest source code with:

svn co https://webserver.svn.codeplex.com/svn/trunk HttpServerTrunk

See also: WebDevelopment