PostgreSQL replication strategies
There are a few tools for master-slave (and master-multislave) scenarios, usually trigger-based. Slony-I has already been mentioned (is stable and solid, but a bit difficult to operate). People having problems with Slony-I wroteLondiste (by Skype team) and PyReplica. Bah, and I just spotted Mammoth has been open-sourced
For multimaster there is Bucardo (note: it is not that polished)or commercial offerings - for example by Continuent or CyberTec.
If you haven't already, I'd suggest a look at the High Availability, Load Balancing, and Replication chapter of the PostgreSQL manual. It gives a clear overview of the available techiques and their features.
Hm, Bucardo is really good and stable, in comparison to the others here. It is as polished as can be a Perl-based replication system, and supports master-slave as well as multi-master replication, with interesting conflict resolution concepts.
If you need simple master-slave rep I'd recommend Londiste, but for the multi-master needs, Bucardo is the only acceptable solution IMHO.