IT Prank
Send out a memo to everyone explaining that all computers have been infected with the new 'Letter G Virus' and that everything is ok, but for the rest of today nobody can type the letter 'G' or else it will cause even more havoc on the company computer systems.

Fictional Awards
Send out memos congradulating people for dumb reasons. Tell them they won an award. Awards for Best Filer or Paper Sorter, etc.

Person Calls
Send everyone an email memo about the new company policy. No more personal calls at the office and only schedules personal visits. Then give everyone their personal visit time slot. Exmaple: Barbara Smith, visting hours 10:57 AM to 11:01 AM

The Clapper Prank
Plug The Clapper into your co-workers computer so whenever someone claps, the computer will turn off and on.

Monitor Settings
Every day, while your co-worker is away from his desk adjust his monitor settings so the brightness goes down 2%. At a rate of 2% a day from 100%...in about 2 weeks the brightness will drop 20%. See how long it takes him to notice the change.

Fun In The Copy Room
Copy some images of obsene content such as gay porn, then place them back in the copier paper bin.

Copier Paper Fun
Type in giant bold letter "Copier Needs Service Soon" and make a few copies of the page. Then place the papers back inside the copier paper tray.

M and N keys
While your co-worker in the next cubicle is away, switch the M and N keys on his keyboard.

Fake Ads in the Company Newsletter
Use the ad section of your company newsletter to post fake bulletins. Place an ad for Windows 98 software still in box for $500 OBO. or maybe a desperate plea for someone to buy your timeshare condo. Be creative!

On Hold Music
Change the 'On Hold' music that your office uses for the phone system. Put it on a Death Metal station or even better, put it on a foreign language station.