Kayak is interesting, especially if you're considering writing a game server as I do from time to time. I don't think its a full web server as much as a http server and request handling framework that maps http calls to C# methods with minimal syntax.
The whole http model was designed nearly 20 years ago for sending text files to a browser. A full web-server does an awful lot more than Kayak usually through 3rd party modules, think cookies, authentication, JavaScript, and all of the complex http protocol that deals with page caching, expiry etc. Kayak has stripped that all away so you can add your own code from the ground up and build something entirely differently but still working of a basic http protocol.
I wonder if it will work out the box on Mono.
- Link to C# Code Library
Yes, it does work out of the box in Mono. Just tested.