Ask for donations (Babel, Webpack). Pro: this works for tools and libraries (not just apps) and you can keep your mission. Con: Companies feel these donations have ambiguous deliverables. There's a lot of mental overhead too (How many projects can one company fund per month?)I don't write software, but I have run various small websites for something like 15+ years. I have always gotten more donation money than ad money off my projects. I switched to a tip jar (last year, iirc) and that further improved my take. (It isn't much, but it beats the figures I have seen quoted by most people when data has been asked for on HN. I also don't get much traffic. For the traffic involved, I think it is pretty good.)
I have also seen Patrick McKenzie talk about the fact that he won't donate money to open source, but if you are willing to write an invoice for him, he is happy to give you money. The reason is that he needs to justify his business expenses on his tax returns and a "donation" is charity that he can't justify to the government, but an invoice for a product he uses in his work is a legit tax deduction. He has talked about how he thinks open source should make invoicing business customers painless. I don't readily have a source at my fingertips, but I bet someone on HN can come up with a link.
A compendium of my own writing about tip jars: http://micheleincalifornia.blogspot.com/search?q=tip+jar