Some people compare open source and free software to oxygen and water. These resources are all around us, and most of us are used to getting them for free. But there are still opportunities for people to make money by selling oxygen or water: Purify it, bottle it and deliver it, and it becomes a cash cow.
There are a couple of very nice articles which discuss about the ways in which you can make money with Linux. Instead of duplicating the information, I would prefer pointing you to the links (thanks to their authors):