I use one of the ringtones that my phone comes with as an alarm because it sucks and I can't download any cool tones. It never fails to wake me up though, I'm a relatively light sleeper.
The only time I overslept was when I forgot to set it to AM instead of PM, so I didn't make it to my 7am class...
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~Halo Everybody run!
http://img9.imageshack.us/img9/6195/furriese.jpg