Gitblit [1] is a nice option if you want an easy install (standalone app or Java WAR) and it runs fine on Windows if that is a constraint you have. It's not as feature rich as some of the other choices out there, but it is actively developed and stable.
We are moving to Gitblit too (from subversion). One of the things I like about Gitblit is that much of the configuration is backed by plain-text files. While that may not be ideal if you have thousands of repositories and a gazillion users, it makes it much easier to back up or change/fix something by hand.
And indeed, the installation procedure is trivial :).
[1] http://www.gitblit.com/