gitjour!
for longer lasting repos, there is git.railscamp.net
Projects being worked on here!
gitjour mit awesome: http://git.railscamp.net/projects/gitjour/repos/mainline
Some useful items mirrored there:
rails: http://git.railscamp.net/projects/rails/repos/mainline (git clone git://git.railscamp.net/rails/mainline.git ) restful_authentication: http://git.railscamp.net/projects/restful_authentication/repos/mainline ( script/plugin install git://git.railscamp.net/restful_authentication/mainline.git )