Drizzle is interesting:
Drizzle: A High-Performance Microkernel DBMS for Scale-Out Applications
Drizzle is a community-driven project based on the popular MySQL DBMS that is focused on MySQL’s original goals of ease-of-use, reliability and performance.
Headed up by Brian Aker, Director of Architecture at MySQL AB. Take a look at the MySQL Differences page and you’ll mostly see features removed and cleaned up, which is great. Designed for high levels of concurrency, targeted to “cloud” applications. Monty and Brian’s posts offer motivation for the project.
Something to keep an eye on.