FreshBSD tracks the source repositories of FreeBSD, NetBSD, OpenBSD and DragonFlyBSD, providing a searchable database of modifications.
It's written in Ruby, running on top of Rails and backed by MySQL, and depends on hand-written parsers of commit emails on the various projects source commit lists.
A new version is in development which will pull history directly from CVS/SVN/Git repositories, and for better searching will be backed by the Lucene search service.
You might be able to view a preliminary version of this at beta.freshbsd.org.