Carbonify offers a carbon dioxide emissions calculator to help you figure out your annual carbon output. Not only does it tell you how much Co2 you're putting out into the world, but how many trees you must then offset per month or year to balance your footprint. Very cool. You can purchase tree offsets from Carbonify here or from Erase Carbon Footprint.com here.
Or, learn which trees offset best and plant you own. Or, for the truly lazy, you can just read a book or two.
Carbonify also provides an offset directory to help you buy green tags so as to assuage your guilt - and all for as little as $2.95! I have a TerraPass myself.
h/t my cyber friend Hilary