James Governor points out that open source software often serves as the personal trainer for its proprietary complements. I’d go one step further and say that open source software is the only way to inject real competition into moribund markets. The biggest reason why Java is pushing .NET in server side development is the open source world. And Mozilla is, of course, the prime example. Opera does good work, but Mozilla has kick started the browser wars. No commercial company (not even Google) could have done that.