I want to add a niche example. The famous Thunderbird addon owl for exchange (https://addons.thunderbird.net/en-US/thunderbird/addon/owl-f...) where you can open the source of the addon and change the licence checks to always return true.

I believe making paid open source code to handle interoperability with something inherently closed / paid (if not outright expensive) is a good approach. Would be users are already used to pay for a related provider anyway, and it's not like the features are so useful without the functionalities of the other provider.

The Ardour DAW is kinda like this. Downloads of prebuilt images through the website are paid, but you can download and build yourself from source. (Most Linux users probably have free access via distro package manager anyway.)

If I remember correctly, other examples of OSS code with premium addons and license check, on the top of my head: Bitwarden, Metabase, EmailEngine

I know there are some open source Mac apps sold for a couple bucks but you can also build for yourself.

That said, I assumed it’s fairly well-trodden mostly because FSF for as long as I can remember approved/recommended something along the lines of selling builds but completely opening the source. I guess RHEL arguably falls into the camp as well, but they sell more than builds.

Ok, got it!

I believe the FSF points about selling software come from a time where you sold the medium (like floppy disks) with the software: you had to allow doing this because it was not practical to spread software without doing this and not costing (too much) to the author.

I don't really think it's practical advice for the present, they were never really business model specialists.

A very cool entertainment one (vs other good examples given so far) that got some nice attention on HN earlier this year [0] is the videogame Shattered Pixel Dungeon. Full GPL3, wonderful fun with a lot of great effort put in and very active ongoing development. You can hack away on it and compile it yourself, but then there are also convenience paid versions on the all the major platforms that seem to do reasonably well.

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0: https://news.ycombinator.com/item?id=39773641