pgpool-II
The replication features of pgpool and pgpool-II work quite differently than the rest of the programs covered here. They are implementations of statement-based middleware. This means that they serve as an intermediate proxy server between your application and the database, translating the statements executed into the appropriate ones for each of the destinations. It can provide synchronous replication as just one of its benefits; refer to http://pgpool.projects.postgresql.org/.
pgpool-II works in one of several modes that include connection pooling, replication, master/slave, and parallel query, some of which can be combined to be active at the same time. It provides a useful toolkit of database scaling features, while suffering a bit in complexity from its lack of focus. Other replication solutions that have a more targeted feature set tend to be easier to set up and work with than pgpool-II.
