gitolite is an access control layer on top of git. It allows you to setup git hosting on a central server, and have multiple "virtual" users (i.e., not "Unix" users) access multiple git repositories, with fine grained access control (read control at the repo level granularity, write control at branch/tag/file/dir level). It has several other features, comprehensive documentation, does not require root permissions, and does not depend on anything except git, Perl 5.8 or later, and any POSIX shell.
GitBlogger is a small tool that allows you to write blog posts in reStructuredText using an editor of your choice, commit your posts to a git repository, and then have them automatically converted to Blogger blog posts when you push your repository to another repository. GitBlogger handles creating, updating, and deleting Blogger posts in response to changes in the git repository and handles renames in your git repository.
Gerrit is a Web-based code review system, facilitating online code reviews for projects using the Git version control system. Gerrit makes reviews easier by showing changes in a side-by-side display and allowing inline comments to be added by any reviewer. Gerrit simplifies Git-based project maintainership by permitting any authorized user to submit changes to the master Git repository, rather than requiring all approved changes to be merged by hand by the project maintainer. This functionality enables a more centralized usage of Git.
git-flow Python Edition is a a pure Python implementation of git-flow. All sub-commands share a common back-end to ensure features behave the same, whatever branch-type they are working on. New workflow behavior can also be added easily by simply sub-classing existing ones. It also includes a comprehensive unit test suite, decent man pages, and HTML docs.