GitPrep, the open source GitHub clone

If you want to run a GitHub-like system in-house, GitPrep by Yuki Kimoto is worth a look.

The project home page includes a
screenshot, but you can also visit a project hosted on a
public facing installation
and can click around.
There are nice diff pages and you can also get a
blame report.

The project is still very young, but looks promising.

GitPrep is written in Perl and it is very easy to install, even on a shared host. It can run
its own web server, use any web server supporting PSGI/Plack, and it can
even run in CGI mode for those shared hosts.

GitPrep is built using the Mojolicius web framework and
Twitter Bootstrap. Its source code is on GitHub
and it is licensed under the Artistic license.