LMDB is an ultra-fast, ultra-compact key-value data store developed by Symas for the OpenLDAP Project. It uses memory-mapped files, so it has the read performance of a pure in-memory database while still offering the persistence of standard disk-based databases, and is only limited to the size of the virtual address space, (it is not limited to the size of physical RAM).
I can see places where this would be useful. In particular, it looks like it would’ve been a better choice of backing store for a small daemon I wrote recently for work. It’s compact nature and the fact it performs MVCC are attractive features to me. There are also Python bindings.