someone at work got me once by changing my .bashrc to include a file that would sleep 0.001 on every command, but every time the file was sourced it would edit itself to increase the time by 0.001 seconds.

A week later I was about to toss my computer off the roof.

Using text-to-speech software and a cron job
At random intervals (several times an hour):
Get volume level
Lower volume level (think ghostly whisper)
Use T-T-S to say something barely audible
Return Volume to previous level

Bonus points if your hands are away from the keybd, or it’s someone else’s machine

In university we used to prank each other by accessing less knowledgeable colleagues’ machines via ssh and running random commands. We would drive them nuts by ejecting the CD tray, or playing random wav files, or popping up silly websites. The lan was not really secure, neither were the passwords.