PostgreSQL is designed to be easily extensible. For
   this reason, extensions loaded into the database can function
   just like features that are built in. The
   contrib/ directory shipped with the source code
   contains several extensions, which are described in
   Appendix F.  Other extensions are developed
   independently, like PostGIS.  Even
   PostgreSQL replication solutions can be developed
   externally. For example,  Slony-I is a popular
   master/standby replication solution that is developed independently
   from the core project.