Here's a some interesting pranks I found when I googled, office humour. British people fucking rock! When you read them, read them with people who annoy you in mind. Makes it even more interesting.
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.
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.
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!
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.
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.