Menubar Countdown is a nifty countdown clock for Mac OS X. Unlike other similar clocks out there, this one sits on the menu bar and shows in real time how much time is left until the clock reaches zero. You can customize what happens after the time is over from the settings window.
From there, you can set the time you want to clock to count down from. The maximum value is 99 hours 59 minutes and 59 seconds. Below this, you can configure the actions to be taken once the clock reaches zero. By default, the app will play a system alert sound, display a notification window and use a text-to-speech engine to read a small message that you can customize. There is also support for Growl, so you can use that to customize your notifications. I liked the text-to-speech feature a lot. It can help you create better notifications and the voice isn't too annoying.
All in all, this app does one thing and one thing only. It puts a countdown clock up in the menu bar where you can always see it, and in case you miss it, there are quite a few notification options to make sure that you somehow are alerted when the countdown reaches zero. It is also completely free.