Synergy binaries (http://synergy-project.org) are behind a paywall. The same goes for Textual (https://www.codeux.com/textual/). They are both GPL and BSD-licensed pieces of software. Ardour (http://ardour.org/download.html) operates the same way.

By paying a small sum you get access to precompiled binaries, some level of support and peace of mind that you are helping developers continue their work in a sustainable way.

Frankly I would like to see this model adopted more often in the FLOSS world.