Hitchhiking is both an ecological and a social form of traveling.
Some might argue that trains are actually more ecological, but by paying for the train you still support unsustainable ways of energy production. In most countries of the world it's possible to hitchhike. In some countries some form of payment is expected, which would reduce the ecological effect, but before accepting a ride you can make clear that you do not want to pay.
Hitchwiki is a wiki dedicated to hitchhiking.