Some kind of GPS-based app for a mobile device that works out how you get to and from work and tries and puts you in touch with others who work closeby and the hours that you do for the sake of carpooling.
Hmm, didn't think I would have gotten as many upvotes as I did...
Anyway, I know this may seem like common sense, but if someone was going to build an app like this, I would suggest that the best platform for it is the iphone simply because Apple actively market their technology as more "green" than other manufacturers.
Using your phone to reduce carbon emissions.