Retrieve forgotten passwords with Keychain Access.
Here is a quick tutorial for something we walk clients through all the time. Say you've forgotten your studio's wireless Airport password. If you checked the "remember this network" option when you first connected wirelessly years ago, your long forgotten password awaits you now in Mac OS X's Keychain Access utility.
Step 1: Launch the Keychain Access utility.
Look in your Mac's Utilities folder to find the often-overlooked Keychain Access utility. The icon is, fittingly, a small ring of keys on a chain. When you launch this utility, you'll be presented with a list of all the passwords you've told your computer to remember over the years.
Step 2: Get info on the password item you need.
Scroll down the main window to find the password item you need. Select “Get Info” from the file menu to see the entry details. You can also double-click the entry to quickly bring up the details.
Step 3: Click on the "Show password" option.
Click on the “Show password” button. You'll need to enter the owner’s main keychain password (usually the same as your Mac's login password).
Voilà, you've uncovered that long-forgotten password.
Source: This tip is inspired by the countless times we've helped clients track down forgotten and missing network passwords. Jordan Bojar also wrote this tip up in his weekly Make Mac Work blog focused on the IT issues facing Macs in Enterprise environments.