"Service Stack" framework

Open source .NET and Mono REST Web Services framework "Service Stack"
is "opinionated" about simplicity and performance.

It is solving similar tasks as Microsoft's ASP.NET Web API, but it is simpler and faster.
It also includes a simple ORM, again much simpler and faster than Micrsofot's "out-of-the-box"
Entity Framework.
It does a few times faster serialization than JSON.Net, another open source tool that Microsoft is now including with Visual Studio.
It can even do SOAP, much better than Microsoft's WCF...

Service Stack is created by a Demis Bellot (mythz) who used to work for StackExchange.com
(linkedin.com/in/mythz)
slideshare.net/newmovie
I would not be surprised that Microsoft ASP.NET "embrace" this new tool in some form soon...

Hanselminutes Technology Podcast - More on Service Stack with John Sonmez:

Pluralsight training

servicestack.net/benchmarks

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Added 2013-02-13:
Podcast interview with Demis Bellot, creator of on ServiceStack @.NET Rocks!