Looking for software to visualize?
The Qualitas Corpus project led by Ewan Tempero at the University of Auckland, New Zealnad maintains a large corpus of open-source Java software for use in empirical studies of software. Contact Ewan for details about the corpus and how to download.
A recent paper: The Qualitas Corpus: A Curated Collection of Java Code for Empirical Studies. Ewan Tempero, Craig Anslow, Jens Dietrich, Ted Han, Jing Li, Markus Lumpe, Hayden Melton, James Noble. APSEC 2010. Paper was awarded best research paper.