I sometimes email open source authors to thank them for their efforts, I don't ask for anything, just an email to show gratification. I'm more inclined to go to this effort for the projects that are highly useful but not popular since as an author of such a project it is possible to have doubts if benefits from it when you go a long time with no outside feedback at all. The few times people have contacted us with projects I've worked on has been good for morale. Sometimes this can lead to other unexpected good things like collaborations later on.
A good example is Karabiner-Elements, a keyboard remapper for macOS. The author and his team has been making sure that it works with every yearly update.
I've sent him a thank-you note, and now follow up yearly with a donation.