Flattr and Gratipay work on this: repeatedly show support/gratitude to people (Gratipay is mostly oriented on software, Flattr is more broad).
Then there's Patreon, but I don't immediately see any software-related stuff there, mostly other creative things (videos/music/drawings etc).
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EDIT: a few examples from Flattr:
NoScript: https://flattr.com/thing/42724/NoScript
Open Street Map: https://flattr.com/profile/openstreetmap
EFF: https://flattr.com/profile/EFF
AdBlock: https://flattr.com/thing/118424/AdBlock-For-Chrome
WikiLeaks: https://flattr.com/profile/WikiLeaks