If you need to use multiple git accounts different ssh keys, for example during work on multiple projects. Here solution.
1) Create or open file:
OSX and Linux: ~/.ssh/config
$ touch config
2) Now create aliases for hosts:
#GitHub work account
Host wrkgithub
HostName github.com
IdentityFile ~/.ssh/path_to_your_work_github_key
#GitHub private account
Host prvtgithub
HostName github.com
IdentityFile ~/.ssh/path_to_your_private_github_key
#Assembla work account
Host wrkassembla
HostName git.assembla.com
IdentityFile ~/.ssh/path_to_your_work_assembla_key
#BitBucket work account
Host wrkbitbucket
HostName bitbucket.org
IdentityFile ~/.ssh/path_to_your_work_assembla_key
3) Use aliases for git urls like this: git@wrkgithub:username/projectnsme.git instead git@
