XMill is a new tool for compressing XML data efficiently.  It is based
on a regrouping strategy that leverages the effect of highly-efficient
compression techniques in compressors such as gzip.  XMill groups XML
text strings with respect to their meaning and exploits similarities
between those text strings for compression.  Hence, XMill typically
achieves much better compression rates than conventional compressors
such as gzip.

WWW: http://www.research.att.com/sw/tools/XMILL/