If you forget your account password or any other password in your keychain, here's how to resolve both issues.

For forgotten account passwords:
1 Insert the Software Install disc that came with your computer.
2 Restart your computer and immediately press and hold the C key to start up from the disc.
3 When the Installer appears, choose Reset Password from the Installer menu and follow the onscreen instructions to choose a new one.
4 Burn this password into your memory.

For forgotten passwords in a keychain:
1 Open Keychain Access (it's in /Applications/Utilities).
2 Click the Passwords disclosure triangle, then select the item that uses the password you've forgotten (for example, select Internet if you've forgotten an email password).
3 In the resulting list, double-click the item whose password you want.
4 In the resulting dialog, select the "Show password" checkbox.
5 Type your keychain password in the resulting dialog (if it's your main keychain, this password is the same as your login password).
6 Click Allow Once to display your password, or click Always Allow to always display your password in Keychain.