Membase is a distributed key-value database management system, optimized for storing data behind interactive Web applications. Membase automatically spreads data and I/O across servers. This "scale out" approach at the data layer permits virtually unlimited growth of transaction capacity, with linear increases in cost and constant per-operation performance. Membase is in production behind thousands of applications, from shared-server deployments at Heroku to dedicated Membase clusters with hundreds of servers at Zynga and NHN.
Metakit is a small, portable database library using a hybrid relational / hierarchical data model with a column-wise internal storage format. For C++, include a single header file and 125 kb DLL or statically link directly. Ideal for desktops, embedded systems, smartphones, and PDAs. Bindings for Python and Tcl. A very mature, stable code base.