Did you know that many (in fact, most) color laser printers are spying on you whenever you print a document? Though you may not have heard the news, the discovery was announced in late 2005. Manufacturers embed a pattern of tiny yellow dots on printed pages. The dots are too small to be seen with the naked eye (especially since they're yellow, see the above photo to see what they actually look like), but under a microscope and blue light they're revealed. The dots are placed in a pattern unique to each printer, and since most color laser printers are purchased through well-documented service providers or direct from the manufacturer, it's simple to track any printed page back to the owner of the printer.
The original idea was obviously to help the government track down currency counterfeiters, since any phony money would be tagged with the yellow dots and would be easily trackable back to the source. But there are also signs of abuse, with the FBI reportedly using the technology to keep tabs on who's printing material for groups like the ACLU and Greenpeace. That's a little scary... and important to remember if you've considering printing a whistleblowing tip or any missive you'd prefer to leave anonymous on a color laser.
If the privacy ramifications of this news bother you, there's some good news. Not all printers have the tracking dots, and the EFF has compiled and updated its list of which ones do and which ones don't. Simply check out this page and look up your printer. (Remember, black and white lasers and inkjet printers don't include the dots.) As the document notes, remember that even if a printer doesn't include the dots, that doesn't mean it isn't using some other method to track your printed pages. If your printer isn't listed here, that means that no information is available about whether that printer includes the dots or not.