Keychain helps you to manage RSA and DSA keys in a convenient and secure manner. It acts as a frontend to SSH-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 RSA and DSA keys.
