Simple Injector is an easy-to-use Dependency Injection (DI) library for .NET 4+  

Math.NET Numerics is an opensource numerical library for .Net, Silverlight and Mono. 

MBrace: the distributed runtime for .NET/mono is now open source 

A simple but effective mini-profiler for .NET, Ruby, Go and Node.js. 

An MVVM framework for .NET that integrates the Reactive Extensions (Rx) framework 

Noda Time  is an alternative date and time API for .NET