One simply has to mention Tokyo/Kyoto Cabinet/Tyrant.
With all the NoSQL Hype Redis and MongoDB get I think these are too often overseen. What's really nice is that you can use them in embedded (cabinet) or server (tyrant) "mode". What's also great about them is that they provide a lot of flexibility, since they are not just one database. Also great that they have official bindings that work. They have tons of features and sometimes an embedded database is really interesting when you don't want (need) to care about the protocol you send it over.