It was also available as open source from Microsoft.
Then it disappeared from asp.net web site.
There is also some projects on code.google.com. This may be original (copy)
larytet-master - Small HTTP server Cassini v35 - JQuant
mpwebstream/Cassini-v35 at master · oxan/mpwebstream (?)
cassini - Microsoft's open source, portable and redistributable web server on .NET 4.0 (fork)
CassiniDev - Cassini 3.5/4.0 Developers Edition - Home: (fork)
New Katana/OWIN solution requires re-write of web applications.
It is better for new projects, and is part of "One ASP.NET" (MVC 5)
Proper "binary distribution" solution for hosting existing ASP.NET (WebForms and MVC) from Microsoft is IIS Express.
This requires running .MSI to install the tool, and to host it in a Windows service.
Download Internet Information Services (IIS) 8.0 Express from Official Microsoft Download Center (supports Windows 8.x)
Why bother when (almost) every Windows 7 and 8.x has IIS as a feature that can be enabled? Because that "enabling" takes 5-10 minutes!
For simple embedded apps that is too much waiting...