The one that comes to mind is PostgreSQL. You know the documentation is good when you can rely on it, and not have to seek out external help in mailing lists,

The one that comes to mind is PostgreSQL. You know the documentation is good when you can rely on it, and not have to seek out external help in mailing lists, Stack Overflow, et cetera. In the case of PostgreSQL, I was able to be productive with it completely offline, with just a downloaded copy of the documentation.