+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 v

+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].