The new stimulus package includes a 10%, upto $500 tax credit for installing Energy Star certified metal roofs by 12/31/2011.  Metal roofs last longer than asphalt roofs and are made of recycled steel.  You can also install them over your present roof. Contractors are also hungry for work, so their rates should be much more reasonable now.

Clearly this type of greening is only appropriate if you need to replace your roof.