There are an awful lot of people who don't understand how one can make money using open source products. It's usually just said the predominant business model is "services" and let it go at that. For those of you who don't know and who want to know, here is a one page, relatively dense, description of JBoss, a company who made enough of a success servicing their open source Java application server to become a desireable target for purchase by larger firms.
This really is worth a read. It's an opinion piece, to be sure, but packed with facts and opinion is clearly delineated from fact.