PostgreSQL was designed from the start to be extensible. For this reason, extensions loaded into the database can function just like features that are packaged with the database. The contrib/ directory shipped with the source code contains a large number of extensions. The README file in that directory contains a summary. They include conversion tools, full-text indexing, XML tools, and additional data types and indexing methods. Other extensions are developed independently, like PostGIS. Even PostgreSQL replication solutions are developed externally. For example, Slony-I is a popular master/slave replication solution that is developed independently from the core project.
There are several administration tools available for PostgreSQL. The most popular is pgAdmin, and there are several commercially available ones.