PostgreSQL is the world's most advanced open-source DBMS, and is increasingly fast, too. It supports all of the features described earlier, such as stored procedures, triggers, and views, but it is not as fast as MySQL as a result. From version 7.4 onwards, there is native Windows support (previously there was not), so Postgres (as it is most commonly referred to) is cross-platform like MySQL.
The Postgres developers have a strong belief in developing standards-compliant software, so they keep fairly stringently to agreed database definitions. The lack of a proper Windows port has left Postgres in MySQL's dust when it comes to speed of adoption, however once users tend to hit the ceiling of MySQL's ability and need functionality that just is not going to be available for at least a year yet, people often migrate over to Postgres. Luckily PHP makes this transition easy.