Digital documents (made out of ones and zeroes) are a lot less wasteful than paper (made out of carbon-eating trees). To minimize unnecessary printing, some people add a reminder at the bottom of emails that says something like "Please consider the environment before printing this."
But the folks at the World Wildlife Fund have another solution. This week, they released a new file format—WWF, of course—which is, essentially, "a PDF that cannot be printed out." Drop by Save as WWF, Save a Tree to download software that will add a "Save as WWF" option to your print menu. Any WWFs you create can be opened by programs that open PDFs—but can't be printed.