Got an email, "Thank you for coverage.py." Opening it, I saw a number of paragraphs. I immediately thought, "they're going to want something, or complain about something."
Nope! It was details of their challenges, and how coverage.py helped, especially the documentation. Ended with "Thanks again for this awesome Python contribution!"
If you like what someone has done, tell them!
@nedbat@hachyderm.io
I have. I just hate doing it through GitHub's messaging system. That said, if that person ever had a real address registered with GitHub, I use an API script to scrape it out so I can try sending direct (which has really surprised some recipients).