I know many TUAW readers keep their Macs on, and awake, over night for a variety of reasons. If your Mac is in your bedroom, then you have probably been rudely awoken by an errant alert from your Mac. Your sleepless nights are now over, because SoundAsleep is here to guard your slumber.
SoundAsleep is a very simple app that sits in your menubar (above you see SoundAsleep's preferences). Simply set the time you want to start muting your Mac, and when you want to unmute it. That's it. SoundAsleep will take care of it.
It would be nice if you could set a number of different 'muting schedules,' but that's why the concept of a version 2.0 was invented.
SoundAsleep is freeware produced by Nullriver.