From Capable Hands Technologies:
Menubar Countdown is a simple countdown timer that displays itself on the right side of the Mac OS X menu bar.
To set the timer, click the display and select the Start... menu item. A dialog will appear that allows you to specify the countdown time in hours, minutes, and seconds. The dialog also allows you to specify which of the following forms of notification you want when the timer gets down to 00:00:00:
Play the system alert sound
Display an alert window
Make a spoken announcement. You can specify the text to be spoken.
Growl notifications are supported as well, if Growl is installed. Use the Growl panel in System Preferences to configure those notifications.