Shameless plug for a tool I developed in order to easily switch git identities based on projects: https://github.com/cquintana92/git-switch-user

After configuring the identities you just need to run

    $ git su Personal
    $ git su Work

Saved me a ton of time.

Shameless plug for a tool I developed in order to easily manage GitHub identities for Git access via SSH:

https://github.com/dolmen/github-keygen

12 years old, but still actively maintained.