One of the best open source .NET projects, in my opinion, is the Castle project. They have an MVC called MonoRail (however ASP.NET MVC is now much better), an ActiveRecord implementation and an IoC container called MicroKernel / Windsor. There are also many other small but useful libraries, such as DynamicProxy.
Currently my favorite .NET library project.
http://www.castleproject.org/
For a large commercial service bus, I prefer to use the Udi Dahan NServiceBus, which is free, open source and extremely efficient (in one example, 100 million long-lasting and 900 million short-lived messages per hour were shown ... a billionth message throughput.)
http://www.nservicebus.com/
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