PostgreSQL eRServer 1.0 + Backplane (Score 4, Insightful)

Two options I haven't seen anyone mention yet are PostgreSQL eRServer 1.0+ (see PostgreSQL news item "PostgreSQL now has working, tested, scalable replication!" from August 28, 2003 or a lengthier press posting "PostgreSQL, Inc. Releases Open Source Replication Version") and Backplane.

eRServer has been in development for over two years, is used in production settings and is released under a BSD license (as with PostgreSQL). It uses a single master/multiple slave asynchronous replication scheme. There are cautions in the release that replication may be difficult to setup.

Backplane seems to be particularly well-suited to clustering data quickly across a WAN. A quote may explain it better:

The Backplane Open Source Database is a replicated, transactional, fault-tolerant relational database core. Currently supported on Linux and FreeBSD, Backplane is designed to run on a large number of small servers rather than a small number of large servers. With Backplane, it is possible to spread the database nodes widely, allowing database operations to work efficiently over WAN latencies while maintaining full transactional coherency across the entire replication group.

Backplane's native quorum-based replication makes it easy to increase the database capacity (by simply adding a new node), re-synch a crashed server, or take down multiple nodes for maintenance (such as an entire co-location facility) - all without affecting the database availability.

I haven't used either yet, but you may wish to give them a look.