kdtree is a simple, easy to use C library for working with kd-trees.

Kd-trees are an extension of binary search trees to k-dimensional data. They facilitate very fast searching, and nearest-neighbor queries.

This particular implementation is designed to be efficient and very easy to use. It is completely written in ANSI/ISO C, and thus completely cross-platform.

http://code.google.com/p/kdtree/