It's not a great place to find yourself, but if you ever lose or forget your password for OS X, you're not out of luck. Weblog AppleDoes details how to quickly and easily reset your password.
All you'll need is your installation DVD. From there, the process is dead simple:
- Insert the DVD to the tray
- Restart and press down the letter ‘C'. It will load the installer DVD
- On the first screen, select Installer > Reset Password
- Select your account and make up a new password.
The AppleDoes post has more than a few typos, so to clarify: If you're using FileVault — the default encryption tool for OS X — and you lose your password, you are out of luck. Also, if the account you want to access isn't an administrator account and you do have the password for your admin account, you can just reset your password by logging into the admin account, opening the Accounts preference pane in the System Preferences, and resetting that user's password. Otherwise, the method above is a great option.
Keep in mind that resetting your password will generally mean you'll lose all the saved passwords saved to Keychain as a security precaution, but it's a pretty good tradeoff.