Product Name: SQL Layer
Key features: The FoundationDB SQL Layer is a SQL engine that runs on top of the FoundationDB Key-Value Store. It inherits all of the properties of the FoundationDB Key Value store including high performance, multi-node scalability, availability, and fault tolerance.
Price: available open source on Github, or free to download & use in production up to 6 server processes at https://foundationdb.com/get
SQL Layer is open source and free to use. It stores its data in FoundationDB's Key-Value Store, however. The Key-Value Store is free for non-production use, and can even be used on up to 6 server processes in production for free. For production clusters of the Key-Value Store greater than 6 server processes, it is $99-$199 per process per month (depending on support level). More info.