For Snow Leopard and earlier
For Lion/Mountain Lion
Boot to the Recovery HD:
Restart the computer and after the chime press and hold down the COMMAND and R keys until the menu screen appears. Alternatively, restart the computer and after the chime press and hold down the OPTION key until the boot manager screen appears. Select the Recovery HD and click on the downward pointing arrow button.
When the menubar appears select Terminal from the Utilities menu. Enter resetpassword at the prompt and press RETURN. Follow instructions in the dialog window that will appear.
The username is the foldername of your admin account. The folder is in the /Users/ folder.