The programs include: a program called Belchbot, which when installed causes the Mac to play belch noises randomly; Gettowork bot, where a voice will yell get to work randomly. There is also Phonebot. This program makes a phone ringing noise in order to get the user to reach for the phone. No telling how many times a person will reach for the phone before he or she realizes what's going on. There are many other sound programs to chose from.
In addition to running randomly, there are some programs that will run in response to an action. Touchy keys will watch for the first key pressed after the Mac boots up, then screams every time that key is pressed.
The favorite among those at Inspired Logic is a program entitled GagMe. This one requires Apple Script. Any time you add something to the trash, the Mac makes a choking noise and spits it out onto the desktop.
Inspired Logic, LLC would like to point out that these programs should be used in the spirit of the fun of April Fools Day and that they should not be used maliciously.
Have a happy April Fool's Day (and we hope you have a job the next day!). :-)