All versions of Mac OS X have a system called Keychain to store passwords. You can open up the Keychain Access program from your Utilities folder to see just what’s being stored, and to manually maintain the database. One particularly useful feature in Keychain Access is the ability to look up forgotten passwords.
It is the Keychain system that remembers the passwords used by iChat, Mail, and many network services. Until recently, when I needed to retrieve a forgotten password, I would locate the item in Keychain Access, double-click it, click the Show Password button, enter my password, and copy the password from there. You can save a few steps by just control-clicking (or right clicking) on the item and selecting “Copy Password to Clipboard.” Just enter your computer’s password, and paste the password where it needs to go.
Keychain Access is also a great place to store secure notes. They function pretty much like the Stickies so many of us know and love, but these are password-protected.