This is a very fast pure C# kd-tree with no dependencies. It also comes with a sample application where you can play with the numbers and perform some nearest neighbor queries.
Since Google Code has removed 'Downloads' for new projects, casual users can get the sample app here. http://highwire-dtc.com/url/kdsharp
This is a C# port of Rednaxela's kd-tree for Java https://bitbucket.org/rednaxela/knn-benchmark
This git repository is a port from the google code project https://code.google.com/p/kd-sharp/