It's also a great example of a technology which is mature and well-tested yet actively growing and improving. Most open source projects have a lot to learn from Postgres.
One of the weirdest is that there is no bug tracker. When things are found to be broken, they tend to get fixed immediately and there is nothing to track. Feature and incremental improvement work seems an arduous process, where you need to own your work and deal with extensive reviewing (no tossing it over the wall for others to maintain).