I've recently put together a resource on LibrePlanet which you're all welcome
to contribute to. It's about making money writing libre software
specifically.
https://libreplanet.org/wiki/Making_money_as_a_libre_software_programmer
Other than writing libre software, selling support for libre software is
already common as Magic Banana wrote. Also there's this talk on having a
libre software IT business:
https://media.libreplanet.org/u/libreplanet/m/building-network-equipment-and-a-business-with-free-software-and-liberated-hardware/
Really though, any business is a "libre software business" if they have
control over their own computing. All businesses deserve and would benefit
from this in theory.
