Keep On Ringing
If you know someone that uses a phone with a hook that presses down when the handset is in the cradle, 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 clueless 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.

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.

Old Toy
Use an old toy that has a button which is used to activate sound. Such as a baby crying. Then tape it to the back of the victims desk drawer, but don't close it completely, leave it open just a crack. When your victim decides to close it, he will hear the annoying sound and wonder where its coming from.

Something is wrong with the copier!
Leave a copier to print 99 copies at 33% resolution on 8x14 paper.

Upside Down Cup
Fill a cup with water about half way. Then place an index card over the cup. Then turn the cup upside down on your co-worker's desk. Finally, carefully slide the index card out from underneath the cup. Whenever your co-worker decides to pick the cup up, he will be drenched in water.

Auto Correct
Microsoft Word has a feature called 'Auto Correct'. It will auto correct any misspelling such as 'TEH' which will automatically be changed to 'THE'. Anyone can go into the tool and change the words. When your co-worker is away from his desk. Change a few of his common words around.

Did I Really Park There?
If you have one of those co-workers that parks in the same spot every day, then try this prank. When he isn't paying attention, take his keys and move his car over just one space.

Format C:
While your co-worker next to you takes a little break, type "Format C:" on his computer screen. Then disconnect his keyboard and mouse. Sit back and wait till he returns.

Glass Cleaner In The Water Cooler
Ok, make sure you don't actually pour any glass cleaner in the water cooler...this is dangerous and very poisonous. A better prank would be to just pour a few drops of blue food coloring in the water and just place a sign on the bottle that reads, "Glass Cleaner"

Jim, you are needed in lumber, Jim to lumber!
If you work in a large superstore and have access to the office intercom system. This prank is great! especially if you are good at disguising your voice. All you have to do is page someone over the intercom telling them they are needed in a department far away from where they currently are. Wait a few minutes then send them somewhere else.

Intercom Prank
This is a great prank to play on a new co-worker. Every office has a code to use the PA. Lets say the code is 10. Well, just let the new co-worker know that he has a call for him on 10. When he presses 10 he will say "hello this is Dave" ...hello? hellooo? Until he realizes he is on the PA intercom

Hide The Chair
When your victim leave the room, hide his chair. When he returns act like you don't know where it is. A great place to hide the chair would be in the opposite sex bathroom

Myra Mains
This is a great prank for a secretary to play on the office jokester. Maybe the secretary needs to get even. Well try this. When your victim is out to lunch, leave him a message that someone named Myra Mains called and needs a call back ASAP. Leave the number to the local coroner's office with the note. Myra Mains, sounds like My Remains.

New Secretary Prank
Call the new secretary and tell her that you are calling from the local phone company and will be running a test on all the phone lines in the building. Let her know that its very crucial that for the next 10 minutes that nobody uses any phone line or else electrocution will result. Then hang up.

Eating Out Of The Trash Can
Put some of your chips in a clean bag on top of the trash can. Then when you and your fellow co-workers are walking past it, grab a handful and eat them in front of them.

Full Blast!
Turn your victim's speaker volume up full blast. He won't know until the next time his computer makes a sound, it will surprise him to death! haha

Office Rumor
Ask your victim if he heard about the rumor going around, tell him people are saying he is getting fired today. It helps if multiple co-worker also tell him the same thing. So plan ahead and get others in on the prank.

Hostage Situation In The Office
Take something that your co-worker needs. Such as his mouse and leave a ransom note in its place. Don't tell him who you are. Just tell him where to leave the money for it.

Courtesy of: PrankSite.com