Slony-I is a trigger-based replication solution which allows you to replicate database tables and sequences asynchronously from one master to several read-only slaves (which can also be cascaded).
Trigger-based means, that each table and sequence which gets replicated has triggers assigned, which will fire whenever the content of the given database object changes. The stored procedures, which are associated in the triggers, will then record the changes and store these in a replication log table. Separate daemons monitor the log table for changes and distribute the changes according to their defined rules.
This approach allows for extremely flexible setups, having different master servers for different tables, but this comes at a price.