Via James Hamilton, on the Windows Live Platform Team, read his writeup.
Facebook has been open sourcing some good software recently, with Thrift. Last week they announced they were open sourcing Cassandra, see the Google code project.
Below is the SlideShare presentation from SIGMOD.
From what I read, and I need to investigate further, it supposedly has a BigTable-like data model running on a Dynamo-like infrastructure. It is written in Java.