Cassini 2 source code is available at Dmitryr’s blog. Although WebDev.WebServer.exe originates from Cassini code, they are not completely identical. Should you need a small (development) web server that does not require the whole Visual Studio installation, use the original Cassini instead of WebDev.WebServer. It runs aspx code and code-behind and is, therefore, very useful for small demo sites and ad-hoc development.
My compiled binary version available here.