If you're not familiar with darcs, it's a revision control system, similar to GNU Arch and git; changes are kept as patch sets, and repositories are branched simply by checking them out. This makes darcs repositories very flexible, and incredibly easy to implement. Static binaries are available for most systems, which makes it easy to install on systems to which you have no administrator rights.
A perl CGI script is shipped with darcs, and provides a web-based repository viewer. It utilizes darcs'
--xml-output switch to create XML, which is then transformed using XSLT. However, there are some issues with the script; it is somewhat difficult to customize, and makes many assumptions about your system (location of configuration files, repositories, etc.). To make it more flexible, I ported it to PHP, using Cgiapp2 and its XSLT template plugin and Phly_Config.
I have released this PHP darcs repository browser as Phly_Darcs, which contains both a Model and Controller, as well as example XSLT view templates. It is currently in beta as I'm still developing PHPUnit2 tests for some of the model functionality, as well as debating the ability to add write capabilities (to authenticated users only, of course).
Update: fixed links to internal pages to use absolute urls.