I used Gittip for a while, and it was a really nice platform, and netted me $5-15/month if I recall correctly. I left after some regulatory issues added an annoying amount of mental gymnastics to actually get money out of it. Later I built my own self-hosted platform for getting tips [1][2]. This tends to net me $20/month consistently, with occasional bursts due to one-time donations. Also, I know of at least one other installation in the wild, it's being used to support some Mastodon instances.
A few months ago, I set up a Patreon [3] where I earn almost $200/month. This is by far the most successful approach I've seen yet, though it rustles my jimmies a bit to be beholden to a proprietary, centralized service. In the end I'm still not making enough to work on open source full time. Thankfully I make enough today to break even on open source (that is, my infrastructural costs are paid for). Hopefully I can figure something out for full time eventually. It's hard.
Many thanks to the folks at Gratipay. It was an uphill battle for sure but they fought the good fight.
[1] https://drewdevault.com/donate