X11R5 jolly japes...
You could grab people's mouse remotely ... and move it just out of window focus as they typed.
Or you could have the cleaners be offended as they walk into the Uni machine room (JCMB anyone?) at 8am to a chorus os Sun 3/50s all playing the When Harry Met Sally orgasm scene through their speakers.
Or xgrab the screen and paste it as the background and confuse the hell out of people.
Or have techy traders grab their mate's screen, read their email and tell the rest of the trade floor their deepest secrets..
Or capture a vocal trader's notorious swearing and be bribed to play it one his machine at the top of every hour. Every day. For a month.
Or even con the traders into thinking that they're running on a windows box using fvwm95.