The most efficient car is the one that you don't have and the second most efficient car is the one not in use. For me its the latter as I caved in and bought a car a few months back. But I still bike because it is sustainable. You can protest oil pipelines and offshore drilling with actions that speak louder than words. Even if you don't agree with man made climate change, we can all agree on the other evils associated with drilling and burning oil. From deforestation, to oil spills, wars, acid rain, smog, etc.
And for the I-don't-care-about-wars-and-environment folk, biking is cheap. Driving costs approx 30cents to a mile, whereas biking is virtually free. Some initial investment and once in a while repairs.
Some practical suggestions if you decide on commuting by bike. Panniers are very handy to avoid carrying backpacks that give you a sweaty back. If biking to work, you may have to take a change of clothes depending on weather, distance and fitness levels. Safety is paramount. I know many people fear getting hit by a car. I wouldn't say unjustified fears, but I think a little overblown. You should ride defensively. You will not win a battle with something 20 times your weight. So don't bother tempting fate. Wear a florescent vest and use front and rear lights to improve your visibility to drivers. Be predictable and try to anticipate the actions of drivers. Don't worry, it becomes second nature after a while. And always wear a helmet. Some practical info for new bikers can be found here.