+1 for Postgres. It's one of the most awesome OSS project: great development team, community, large codebase (which is well organized and very modular), has a very good code and end-user documentation, always working with a significant number of important computer science problems and concepts (ranging from easy to very complex ones).

[Disclosure: I'm not a Postgres contributor myself. In the past, I had been working on Postgres core and related projects at EnterpriseDB (a Postgres company) for 5+ years].

This. I actually joined HN years ago just to praise Tom Lane for his outstanding support (I hope I remember this correctly).

Ditto. The community seems to have a remarkable ability to take the high road.

Definitely agree. The PostgreSQL Community began my serious involvement with Open Source Software, about 15 years ago. It's one of the best groups I've ever seen, due to their patience, friendliness, encouragement of others, and willingness to share info/experiences for mutual benefit.

It doesn't hurt that being very good with PostgreSQL is useful on a CV too. ;)