FoundationDB
Open source, distributed, transactional key-value store
...FoundationDB provides amazing performance on commodity hardware, allowing you to support very heavy loads at low cost. FoundationDB has been running in production for years and been hardened with lessons learned. Backing FoundationDB up is an unmatched testing system based on a deterministic simulation engine. Developers on an OS for which there is no binary package, or who would like to start hacking on the code, can get started by compiling from source.