XMLPPM is a data compression program that compresses XML files from 5 to
30% better than any existing text or XML-specific compressors. It is a
combination of the well-known Prediction by Partial Match (PPM) algorithm
for text compression, first described by Cleary and Witten in 1984, and an
approach to modeling tree-structured data called Multiplexed Hierarchical
Modeling (MHM) that I have developed.

WWW: http://www.cs.cornell.edu/People/jcheney/xmlppm/xmlppm.html