Kuku Klok is a simple Flash-based alarm clock
Kuku Klok is a cute little Flash alarm clock. The interface couldn't really be simpler; you use the clock on the left side to set your desired wake-up time, select a sound (a cockerel by default, but you also have a classic clock, insanely irritating electronic chirping, metal guitar, and a military bugle), and hit "set alarm".
Then when time comes, assuming your computer did not shut off during the night, the alarm clock would wake you up. That's all there is to it, really. It works even if you're no longer connected to the internet, since it's just a small Flash file. Theoretically, you could even save the SWF on your computer and just use it locally. It's simple, and can be effective for occasional use (on the road, for example).