I maintain a moderately successful JavaScript library and there are a few things that absolutely make my day: people sending me an email telling me how they're using it (I see dependent projects on NPM but I love hearing about cool things!), people saying thanks, and people sending me any donation. I've had a handful of people send me a few bucks over the years and I was absolutely floored each time. In total I might be able to buy myself a beer with the donations but it means a surprisingly large amount knowing someone is willing to go through the hassle to send you money.