If you ever installed Web Matrix (0.5 or 0.6 - it will never make version 1...)
you will already know what Cassini is... a lightweight C# web server that enables ASP.NET applications to be run without IIS.
MSDN explains Client-side Hosting with Cassini and how Cassini is used in MS-CRM.
"What use is that?", you ask... well despite the fact that it appears to be just a cute open source project, Microsoft has implemented it (well, some form of it) in Visual Studio 2005 as the primary way for developers to run ASP.NET applications without having to install IIS.
You can also use it, as Scott Hanselman has, to cobble together an NUnit ASP.NET tester using Cassini...
FYI, the name "Cassini" sprung from the code name for Web Matrix (which was "Saturn")... Cassini is of course on of Saturn's moons. More code-names decoded here.