A collection of index based algorithms for comparison purposes. Also contains RStarTrees for indexing and the DBSCAN algorithm. Yet, this library is not intended as a library and really is complicated to use (see http://elki.dbs.ifi.lmu.de/wiki/HowTo/InvokingELKIFromJava).
An RTree implementation by the University of Michigan. It seems to be part of a bigger library 'EZFilter'. The implementation realizes the Map interface with the additional method '#getOverlapping(RTreeKey)'. Using this as a library seems not to be intended.