Subversion is a version control system. Originally designed to be a compelling replacement for CVS in the open source community, it has far exceeded that goal and seen widespread adoption in both open source and corporate environments. The Subversion project produces Subversion's core libraries (written in C), a fully functional command line client (svn), repository administration programs, API bindings for various languages (Perl, Python, Java, Ruby, etc.), and various additional tools and scripts.
CVSSearch searches for code fragments using CVS comments. Specifically, it takes advantage of the fact that a CVS comment describes the lines of code involved in the commit and that this description will typically hold for many future versions. In other words, CVSSearch allows you to better search the most recent version of the code by looking at previous versions to better understand the current version.
The FILTR (File Inventory for Loading, Transfer, and Recovery) automates CVS functions for personal use, allowing a user to maintain a watch on any directory, mirroring it in an intermediate working directory and taking care of all import, add, delete, and update calls to the CVS repository. No changes or additions are made to the directory under watch. The user can revert a watched directory to any previous saved state with a button click, or call up one or several past versions of a file. Groups can also make use of the same repository for file sharing.
Katie is a revision control system, somewhat like a cross between CVS and NFS, that was inspired by Rational ClearCase. The three most interesting features are that the repository is mounted as a filesystem (rather than being copied to a local workspace), that all versions of all files (even deleted ones) are accessible through this filesystem (so the "katie diff" command is a convenience rather than a necessity like "cvs diff"), and that directories are versioned (just like files are). It is functional enough to be self-hosting, but there is much work still to go before it will be a generally useful tool. Features that are implemented already include VOBs, elements, branches, dynamic views, view-extended pathnames, config specs (including auto-make-branch rules), labels, hard links, and symbolic links.