The general download location for Git releases is http://www.kernel.org/pub/software/scm/git/. You can also use one of many kernel.org mirrors.
For people who prefer precompiled packages, these are available:
Daily snapshots of the main Git development branch are available at
codemonkey.org
(thanks to Dave Jones).
If you already have Git installed, you can get the latest development version via Git itself:
git clone git://git.kernel.org/pub/scm/git/git.git
If you have problems connecting (Git uses port 9418), you can try to access the repository over the HTTP protocol:
git clone http://www.kernel.org/pub/scm/git/git.git
(this method is considerably slower but works even behind firewalls and such).
You can also always browse the current contents of the git repository using either the gitweb or GitHub web interface.