[poops]

There was a dirty mouse program that would get installed anytime someone had their computer unlocked. Every few minutes you'd hear someone get pissed off and banging their mouse on the desk.

Another one we did was take a screenshot of a 404 page, and then randomly show that instead of the site that they were working on, but only for their IP.

A quick and easy one was to just crank someone's speakers all the way up, for the next time they play music.