A thin layer over MySQL, to access data in JSON format. Supports connection pooling and load balancing.

Here you'll find few interesting use cases for MySQL Proxy such as logging, query injection, and even custom results generation.

This article describe how Skype is planning to scale their Postgre SQL installation on 1 billion users. They use PLProxy and PgBouncer to make horizontal partitioning.

Interesting flame about CouchDB and other non relational databases.