When using Git via SSH with services such as GitHub and Gitorious, it can be useful to use specific / different ssh keys than your default.
This is accomplished with an entry such as the following in your ~/.ssh/config:
Host some-aliasIdentityFile /home/username/.ssh/id_rsa-some-aliasIdentitiesOnly yesHostName git.example.comUser git
You then specify this remote as follows in .git/config:
url = some-alias:project/repository.git