Laurent Balmelli from the neutral country of Switzerland has some C++ code to do some interesting quadtree processing. He would like you register if you are going to use the code, but this is not mandatory. This code is being used by over 100 registered users, and has at least been accessed by as many as 2500 others.

http://lcavwww.epfl.ch/~balmelli/software/quadtree/index.html