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.
This is a C# port of Rednaxela's kd-tree for Java: https://bitbucket.org/rednaxela/knn-benchmark
Since Google Code has removed 'Downloads' for new projects, casual users can get the sample app here: http://highwire-dtc.com/url/kdsharp