I found a nice simple Mac prank the other day which I tested with some success in our showroom.
Log into a machine you can see (and preferably hear) via ssh when someone else is near it. You can make sure its volume is turned up using this terminal command:

sudo osascript -e "set Volume 10"

10 for max volume, 0 is mute.

You can then freak people out by using the 'say' command:

say "anything you like"

Its good fun when you get someone who is less of a geek and can't work out what is happening.

Next trick is to broadcast the AV from an iSight on the client over the network without showing the image on screen or playing the audio through internal speakers.