Source code repositories
Subversion
Subversion is the tool the FreeBSD Project uses for keeping its sources under control. Every change (with an accompanying log message explaining its purpose) is stored. It can be easily viewed from the web interface mentioned below.
In June 2008, development of the base system moved to a different version control system, Subversion (SVN for short). The web interface is available for browsing the repository. All changes are also exported back to the CVS repository.
In May 2012, the FreeBSD Documentation Project moved from CVS to Subversion. Unlike the base system, the documentation SVN repository is not exported back to CVS. There is a web interface available for browsing the contents of the FreeBSD Documentation Project SVN repository.
In July 2012, the FreeBSD Ports tree moved from CVS to Subversion. There is a web interface for browsing the repository. The Ports tree is also exported back to the CVS repository. It will cease to be exported early 2013.
Legacy - CVS
CVS (the Concurrent Version System) was the tool that the FreeBSD Project used to use to keep the sources under control.
The old web interface can be accessed at the cvsweb instance .
Other options
CTM if you are looking for very low overhead, batch-mode access (basically, patches through email).