Depending on how much money you have to spare, you can solar power your house. If you make more power than your house requires, your local electricity company will buy that green power off you to power something else... so that's almost getting 2 ticks, and a couple of dollars at the same time
Rain water tank - I'm not sure if they can hook it back into your house for use, but it can be used on your vegie patch
Grey water - you are saving all that water that would usually go down your drain, and use it on your garden. Unfortunately there are some councils that don't see the positive side to grey water is actually not permitted in some areas.
Growing your own vegie patch, and for you vegetarians - having your own chooks. Even if you're a vegan - having chooks will save on garden pesticides as they will look after that for you.
For those of you at school, or in an office environment, and need to print out assignments, reports, etc... Reflex have a carbon neutral copy paper. It does cost more, but it comes with a government seal of approval.