Until now we've been aware of 2 excelent artificial intelligence frameworks written in C#. These are AForge.NET and its successor Accord.NET. The both include a lot of algorithms for solving wide range of tasks.
Yesterday we've discovered that Microsoft has published as an open-source project their Computational Network Toolkit in order to speed up advances in artificial intelligence and made it available to a broader group of developers.
The sources written in C++ which scales good and uses GPUs. This gives a competitive advantage to CNTK, see more details here.
Although the main aim of such development was speech recognition, the CNTK contains a Neural Network framework that may be used for other artificial intelligence tasks.