A bicycle, naturally. However, as you may expect, this is still quite rare and most of Americans prefer (or have to) drive. This recent poll on Slashdot shows the current trend:
Slashdot may not be the most representative source for the entire country, since it is mostly read by tech workers (engineers, IT people, scientists, etc.) but it probably shows the real life situation quite well. Most of us drive to work (46%), while public transportation is essentially as popular (16%) as walking or cycling (19%). Only a minority can work from home, remotely (7%).
It would be interesting to see this poll done in other countries. Here, in U.S. we chose to live so far away in the suburbs that we became dependent on cars, as there are no other means of transportation available nearby. I am pretty sure the 16% and 19% bars would be much longer in Europe where people generally live closer to city centers.