keychain helps you to manage ssh keys in a convenient and secure manner. It acts as a frontend to ssh-agent and gpg-agent, but allows you to easily have one long 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.
