There seems to be a host of these micro web frameworks on .NET now:
Nancy: Here is a HanselMinutes podcast with the creator Andreas Håkansson
Nina: "Nina is a web microframework for the .Net platform, inspired by Sinatra"
Kayak: "Kayak is an asynchronous HTTP server written in C#"
Manos: Interview with the creator Jackson Harper on Hearding Code
Jessica is "a micro web framework written out of curiosity in .NET, inspired by Ruby's Sinatra."
There is good blog article by Mike Hadlow where I picked up the above links and he goes into good detail on each. Just thought I'd shoutout these projects, it's exciting to see such development in the .NET world.