If you want something simple and do not wish to install any additional gadgets to your computer, for instance, Google Desktop (it has the alarm clock feature), Naked Alarm Clock is always a good choice for you to set an alarm.
It's good for those who let their computer on whole night while sleeping (not healthy to sleep with your computer, though). It will make a great alarm, you can choose from 4 different alarm tones. The interface is quite straight forward, need me say no more. :)
A great alarm tool if your alarm clock is temporarily out of service :D
