File Cabinet Prank
Put a small animal in the file cabinet. Make sure its a cabinet that is opened multiple times daily.

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 fire 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.

Phone Label Swap
Change the speed dial labels on office phones. It will piss off your co-worker when he is trying to call tech support but instead speed dials the courier.

Calculator Fun
Place some clear tape over the calculator to make the numbers very fuzzy.

Discontinued Award
The next time someone at the office wins an award, send out a memo to everyone that the award has been discontinued.

Porn CDs
Relabel the data CDs so they say 'Porn' on them.

Self Recognition Prank
Without disguising your voice, compliment yourself over the office intercom. Compliment yourself on a just completed task or give yourself the rest of the day off.

Screen Capture
Take a screen capture of someone's screen. It would work best if they have a program open. Then save the screen capture as an image and use that as their desktop background picture. It will confuse the heck out of them when they see their desktop icons over their microsoft word.