HyperDB

HyperDB is a very advanced database class that replaces a few of the WordPress built-in database functions. The main differences are:
* HyperDB can be connect to an arbitrary number of database servers,
* HyperDB inspects each query to determine the appropriate database.

It supports:

• Read and write servers (replication)
• Configurable priority for reading and writing
• Local and remote datacenters
• Private and public networks
• Different tables on different databases/hosts
• Smart post-write master reads
• Failover for downed host
• Advanced statistics for profiling
• WordPress Multisite

It is based on the code currently used in production on WordPress.com with many MySQL servers spanning multiple datacenters.