Have you heard of
Drizzle? As detailed
here, it's a lightweight, open source SQL database for the cloud and the web, being designed for massive concurrency on multi-CPU/core architecture. If that sounds a lot like MySQL, it should. Drizzle's code is derived from MySQL--basically a fork of the MySQL 6.0 source code. However, it's stripped down in several ways deliberately, or refactored, for solutions to some of the problems that MySQL runs into. Brian Aker, who is an engineer at Sun, and behind the project, has a
post up about it. He cites speed, simplicity and less overhead as benefits of Drizzle over MySQL. The project is on Launchpad
here for developers, although you can't downlaod and use it yet. Here's more backstory.