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 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.
(From http://www.funtoo.org/wiki/Keychain)

Install keychain:

Ubuntu/Debian:

sudo apt-get install keychain

Configure keychain:

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Add this to your ~/.bashrc:

keychain ~/.ssh/some.key ~/.ssh/another.key
. ~/.keychain/$HOSTNAME-sh

For every key you need to add the public key in the same directory with the filename $keyname.pub.
For example if you have a keyfile named id_rsa you need the id_rsa.pub in the same directory.