A Netherlands outfit named SPRANQ has created a special font that when used claims less ink usage on paper.
The font is a basic sans-serif with a twist. Inside each letter are open circles where no ink is used.
Ecofont is available as a free download and you can grab a copy on the SPRANQ website. If you prefer to not use this font, you can always do the tried and true when printing, which is to use a small font size and print on double sided paper.