Inspired by something I read this morning, I thought I would make a mention of the Koha developers who are employed by a library. Off the top of my head (and if I have missed some please let me know in comments) I can think of.
Owen, Marcel, Dobrica, Mirko, Tomas, Mathieu, and the Koha Lyon team. All of these developers have committed numerous patches to Koha, and not just bug fixes but a lot of new features too. I think this is quite remarkable and a good example of doing it right, take some of the money you save in licensing fees and take control of your own future.
Oh, I have just remembered Steve Tonnesen who wrote the first web based circulation, and was an employee of the Coastal Mountain School district. Also as Paul pointed out in the comments I forgot Arnaud, Hélène and Bruno from SAN-Ouest Provence.