Facebook has stepped up its contribution to the open source community with the release of the
Cassandra project on Google Code. The location is mildly ironic: Cassandra is roughly an open source alternative to Google's internal BigTable storage system. (BigTable itself isn't open source, though you can gain access to it via the AppEngine
DataStore API). Cassandra isn't for everyone, but if you're dealing with scalability issues arising from database concurrency bottlenecks, it's something that you should know about.