We all forget passwords or they momentarily elude memory. To find a password open Keychain Access.app in the Utilitiesfolder nested in the Applications folder. Once there, scroll through the list of keys until you find the one that you’re looking for. Double click on it and check the box that says, “Show Password.”
You will have to authenticate with your user credentials, your forgotten password will be displayed in the text box.
ref. Mac 101: Retrieve your Keychain passwords, TUAW Sep 25th 2008