OpenGrok is a fast source code search and cross reference engine.
It helps you search, cross-reference and navigate your source tree.  It can
understand various program file formats and version control histories like
Mercurial, Git, SCCS, RCS, CVS, Subversion, Teamware, ClearCase, Perforce
and Bazaar.  In other words it lets you grok (profoundly understand) the
open source, hence the name OpenGrok.  It is written in Java.