as already mentioned there's no in core multi master replication, but other then the already listed solutions, you can have look at :

http://postgres-xc.sourceforge.net/

the project is sponsored by Enterprisedb and NTT

a quote from the home page:

"Postgres-XC is an open source project to provide a write-scalable, synchronous multi-master, transparent PostgreSQL cluster solution. It is a collection if tightly coupled database components which can be installed in more than one hardware or virtual machines."

[edit: sorry for the noise I did't notice that jeff davis already comment about postgres-xc]