Keep On Ringing
If you know someone that uses a phone with a hook that presses down when the handset is in the craddle, then tape that part down. When the person tries to answer a call it will just keep on ringing and ringing.

20+ cups on desk
Fill about 20 or more paper cups up with water, place them next to each other on your victims desk. Then staple them all together, stand back and watch as he/she arrives at work and is clueeless about how to go about removing the cups off the desk.

The Zoo called
Call someone at the office while they are away and leave a message saying Mr. Lyon (or Mr. Behr) called and would appreciate a call back. Then leave the phone number for the local zoo on the answering machine for this person to call.

Mouse Trap
If you know someone that uses an optical mouse, place a small post it note over the eye of the mouse. When your victim tries to use their mouse it won't work. When they decide to look underneath it, they will see a small posted that reads, "April Fools"

Ypto Ypto...errr!!! TYPO!
Be carefull with this one, as to not ruin the keyboard. All you have to do is simply and very carefully remove a few keys and switch them around. If your victim is one of those "look up and down" typers. He or she will be very confused while trying to get work done.

Where are those paperclips?
Make a few copies of paperclips then put those copies back inside the paper tray. Whoever uses the copier next will get their copies with those paperclips on them and will look all over trying to hunt them down.

Stale Donuts
About a week before the prank, buy a box of donuts. Keep them in the refridgerator with the top open so they can dry out. On the day of the prank, place the box of donuts near the office coffee pot so anyone with a morning sweet tooth can indulge.

Pick Up The Entire Phone
Wrap a very small piece of wire around the phone cord so both ends are tied together. When the phone rings and your victim goes to pick up the receiver, he will up with the entire phone base with it.

Confusing Drawers
Before your co-worker arrives at the office remove his desk drawers and switch them around. He will definitely scratch his head over this one. Hint: In case you cannot remove the drawers, just remove the items and swap them

Pen Swap
Take all of your co-wrokers pens and replace them with your pens that your glued caps on to the night before. He will not be able to take the caps off to use the pens.