I second this, the Postgres contributors are consistently setting the bar for the rest of OSS projects out there, it's consistently been my favorite part of the stack for a long time now.

Don't forget about their phenomenal #postgresql channel on Freenode. The folks working on Postgres have been gracious enough to patiently answer my not always fully baked questions for the past 5 years on there, they're a bottomless treasure trove of best practices and pragmatic advice.