I’m probably the last one to know about this gem, but it sure has made my life easier. I have like 4 or 5 random ssh keys to keep track of that map to various users on various systems. Up until know this has meant passing the write key file with the -i option to SSH, but no more; ~/ssh/config makes life much easier. All you have to do is define which sites go with which user and key and ding ding life is good.
Host example.com User root IdentityFile ~/ec2/example.pem Host example2.com User gitosis IdentityFile ~/.ssh/id_rsa