You can find various option to earn bitcoin without buying. Few of which is often suggested are gambling, signature campaign, faucets, trading.
Other better options to earn bitcoin is to get freelance jobs from the services section of our bitcointalk forum.
I do agree that many ways such as faucets or signature campaign which can be applicable to earn bit coins but it is not possible it seems without buying a bitcoin since it would take lot of efforts and hard work to earn such high value of being able to get a bit coin.