One great way to help the environment is to have less people driving, and employers can help encourage that.
Some suggestions:
Rewarding carpoolers with closer, reserved parking spaces.
Offering discounts on public transportation (most metro areas have programs for employers to be able to do this)
Offering the option to work remotely from home (added benefit: fewer call-offs for bad weather, car trouble, etc.)
I'm sure we could think of other ideas, but I think these would be a good start.