There’s a new Git GUI on Windows: https://github.com/kaisellgren/Git-GUI. It uses libgit2 and .NET 4.5.
GitHub for Windows is also libgit2+.NET: https://github.com/blog/1127-github-for-windows, and interoperates with the GitHub site. However, since libgit2 isn’t yet feature-complete, GitHub for Windows also uses msysGit where necessary.
Since libgit2 has pluggable backends, people have used it to store Git repositories in databases: https://deveo.com/blog/2013/06/19/your-git-repository-in-a-database-pluggable-backends-in-libgit2/. Some existing backends include Memcached, MySQL, Redis and SQLite.