Keychain helps you to manage SSH and GPG keys in a convenient and
secure manner. It acts as a frontend to ssh-agent and ssh-add, but
allows you to easily have one long running ssh-agent process per
running ssh-agent process per system, rather than the norm of one
ssh-agent per login session.

This dramatically reduces the number of times you need to enter your
passphrase. With keychain, you only need to enter a passphrase once
every time your local machine is rebooted. Keychain also makes it
easy for remote cron jobs to securely "hook in" to a long running
ssh-agent process, allowing your scripts to take advantage of
key-based logins.

Keychain also integrates with gpg-agent, so that GPG keys can be
cached at the same time as SSH keys.

Additional documentation for Keychain can be found on the Keychain wiki
page.