iTunes Alarm

iTunes Alarm transforms iTunes into a simple and customizable alarm clock, waking you up with your favorite music.

Create as many alarms as you need and then put your computer in sleep mode; iTunes Alarm will make sure to wake it up when your alarm goes off. You can also use iTunes Alarm to fall asleep while you listen to music. All you have to do is set when it should stop playing the music.

If you're looking for a similar application that can also turn on the computer when it's completely turned off (not just in sleep mode), then you should check out iTaf.