Saw this on digg today. It's an old trick, but still great. If your school or office has HP printers, try using this tool to change the settings. I had a lot of fun today changing the LCD screen display to say "INSERT COIN" or "CHANGE WHITE TONER." There are some very confused people today.

How to use:
1) Run it
2) Enter IP of printer (leave port at default usually)
3) Click the rightmost icon
4) Change message!

To find the IP of the printer, go to printers & faxes, right click properties of the printer, then look at the ports tab.