https://github.com/rweeks/util/blob/master/src/com/newbrightidea/util/RTree.java -
LGPL implementation of R-Tree by Russ Weeks. It looks very simple and clear and not dependent on external libraries.
http://www.mischiefblog.com/?p=171
http://www.mischiefbox.com/blog/uploads/rtree.jar
LGPL implementation of R-Tree by Chris Jones. Another simple and clear solution.
http://www.khelekore.org/prtree/
CPL 1.0 implementation of Priority R-Tree by Robert Olofsson
http://jsi.sourceforge.net/
LGPL - project aims to maintain a high performance Java version of the RTree spatial indexing algorithm.
