Well, in two fields I'm familiar with -- amateur radio and software -- lots of people do both "amateur" and "professional" activities. I know lots of people who are electronic designers or who build, deploy, and maintain communications systems as their livelihood; but then in their free time they may do amateur radio emergency and public service work; design, scrounge parts for, build, and test equipment for microwave communication; or just talk on the radio while commuting. Similarly, a lot of programmers who work on commercial applications and systems then turn around and pitch in on an open source project.