For April Fool's this year, I designed this little electronics project. It is a 555 timer circuit that beeps every few minutes. The beeps are just infrequent enough that it is hard to locate but still be really annoying. It is basically a DIY Annoy-a-tron. The main advantage of this project is how small it is. It can fit just about anywhere and even be attached to vertical surfaces with just a small piece of tape.
As with all pranks, it is very important to choose your victims carefully. Someone who is paranoid might mistake it for a bomb or something ridiculous like that.
For step by step insctuctions for this project, check out my Instructables page: http://www.instructables.com/id/Miniature-Beeping-Circuit-Prank/