There’s a handful of us that do 50 for FOSS: https://50forFOSS.org
tl;dr: on the first Friday of the month we each pick a FOSS (free/open-source) project and give the maintainer $50.
Thanks and encouragement is great too. As a small-time open source maintainer, it seems awareness has been spreading over the last few years and people are going out of their way to be kind and respectful when they raise issues; it really makes a difference. But financial sustainability and community ownership are separate and arguably more essential issues if we want FOSS to survive over the long term.
I did have one maintainer turn down the $50 and ask me to donate it to UNICEF. It’s all the same to me as long as it makes the work more sustainable for them.