I just started using the sound of a compact Mac making the standard "short beep" sound from the pre-System 6 days. Why didn't I think of this before?
Part of the charm of this sound is the way it resonates in the case, so instead of recording the sound from Mini vMac, I sampled an actual compact Mac making the sound. :-) The thought of my aluminum 2011 MacBook Pro beeping at me with this oddly plasticy-and-resonant beep just cracks me up.
Download a copy of the sound and dump it in /System/Library/Sounds.