If you're logged in with multiple shells on a machine and want to reuse the SSH agent between them, there's an easy way that doesn't involve anything but a line of BASH.
In your first shell, run ssh-add as normal to add the keys you need:
$ ssh login.example.com
$ ssh-add
Then, in the other shell(s), re-use the SSH agent from the first shell with:
$ ssh login.example.com
$ export SSH_AUTH_SOCK=$(find /tmp/ssh-* -name "agent.*" -user "$(whoami)" | tail -n 1)
Nice, secure and simple.
