When two (ore more) people are working a desks that are put next to each other, switch their telephone cords. With all those cables lying around it will take some time before they find that one out!

Put a peice of onion or a clove of garlic inside the mouthpiece of a phone. Give it some time for it to fester and build up a strong odor. Then call them and keep them on the phone for as long as possible.

It is allways a good habit to lock your computer before heading off for coffee or a smoke. When someone forgets and leaves a Word document or an email open, type a single word somewhere in the text. “f*ck” or so will do nicely. They’ll never notice and send it out.

Get a hold of someone's cell phone and change the greeting banner to say "NO SERVICE". Many cell phones have greeting banners on them that you can personalize to say whatever you want them to and it stays on there when you're not using your phone. Also, when there is no service where you are, most cell phone companies have a banner that pops up on your screen saying "no service".

Take a can of non gel shaving cream, and put it in a freezer. When it is frozen remove the bottom of the can and put it in co worker's drawer. When it melts it expands and explodes all over everything.

Go into MS Word or similar program on co-worker's computer, and add an entry to the AutoCorrect feature. This is a very simple prank that will send the novice user into a frenzy. Configure the AutoCorrect option to replace the word "the" with the phrase "you suck!". They will usually panick and start scanning for viruses.

Take clear tape and tape the underside of the mouse. Make sure you take the sticky end of the tape and apply it to the bottom of the mouse so it locks the ball in place. The victim will most likely check the connections in the back, reinstall drivers, reboot, etc., before they realize what has happened.

Do a "Print Screen" of the user's desktop, and then paste the image from the clipboard to a photo program, and save the image as a bitmap. Then, set the 'snapshot' of their desktop as the actual desktop wallpaper. (You'll have to hide the Windows status bar, and move all their desktop icons into a folder, which you can hide conspicuously in the corner or something.) The user will see their desktop as always, but everything on it will appear to be frozen when they try to click on it...sending them into a rebooting and virus scanning fit!

Try to find a very obnoxious CD laying around. Preferably a reggae or rap CD. Pop it in their CD ROM. Put up the sound full blast by double clicking on the volume control on the bottom right. On normal configurations the audio CD will autoplay when windows first starts up. The person starting up there PC in the morning will definitely be embarrassed.

This is for that special person you just cant stand in the office, the one who talks on the phone all day with their boyfriend/girlfriend and gets personal e-mail all day. Go into their e-mail and change their defaults to autmatically "blind carbon copy" their boss or supervisor. Heads will roll!

Change the coffee in the office coffe maker to decafe. Wait about three weeks(or untill you think everybody has gotten over their caffine addiction)and switch to expresso!

Try "password securing" someone's screen saver. First I suggest changing the screen saver to "scrolling marque" and inserting your own word or phrase, "Mr. Jones (president or supervisor) eats SHlT" or something to that effect.

With someone who is on the phone a lot during work - This works if you have phones that the handset comes apart. Take the handset apart and put scotch tape over the mouthpeice inside. They can still be heard, but they have to talk loud to be heard. The next day take it off, and put it in the earpeice. Usually they will be yelling to the other person on the line the next day, and won't be able to hear them. When they complain about the phone, and get a replacement, do it on the next phone. After about a week you will notice the calls to be down considerably.

Depending where you are at you may have a cafeteria in you place of work. Every week most of them put out a menu so you know what they are serving. Usually it is done on Word or Excel, and not extremely fancy. With a little work, matching fonts, and images you can make your own menus, and post them by your desk. We had one co-worker avoid the cafeteria for 2 weeks because of the selection "fish head stew" etc... before he caught on. Works great with picky eaters.

My absolutely most favorite prank I have saved for last. It is so simple to do and yields such nice results. Simply pop out the 'm' and 'n' key on someone's keyboard and reverse the two. Any flat tool will work. Just pry it with little pressure and they will easily come right off. Then just sit back and watch the confusion.