While I'm the project lead for one of the more popular Erlang web frameworks (http://nitrogenproject.com), my main focus is on my sports league management system[1] (http://bracketpal.com), which runs on Nitrogen.
As a result, I can justify spending "free" time working on open source because at the end of the day, it improves my main products.
It's an interesting way to piggyback the so-called "unicorn" project onto a product (the design of which I get to control). Granted, it'd be nice to just get paid to work on Nitrogen all the time, since that work is generally more mentally stimulating, but I can't complain.
[1] I know the landing page is a total dog. It's the current project to fix that up to something not terrible.