Ecofont

I love this one: By changing your font, you can use 20% less ink when printing documents. Ecofont has tiny holes in it that are imperceptible at 10 point and below (I can see them if I look really hard when printed in 11 point on my pretty low-end HP laser printer). The font was easy to download and install (clear instructions are provided) -- the only trick for me was looking for the font under its full name of "Spranq eco sans" rather than "ecofont" in Word's font list. It looks a little lighter on-screen, too, so I'd say it's best for people that actually print their documents, not just read them on-screen.