Signing a tag with GPG:
    http://kerneltrap.org/mailarchive/git/2009/4/3/5717793

Rebasing:
    http://blog.woobling.org/2009/08/git-rebase-for-impatient.html
    http://blog.woobling.org/2009/05/git-rebase-considered-awesome.html

Git internals:
    http://www.newartisans.com/2008/04/git-from-the-bottom-up.html