Distributed key-value stores are a hot topic of the moment. It appears that there are many situations where the current relational database paradigm is not suited. Situations where simple key retrieval is enough - no joins or relations between tables. And also distributed: this is more and more a requirement, and the traditional databases are not designed with distribution in mind, although there are various strategies in place to cope (sharding, duplications like master/slave setups).

This post is a good write-up of the more well-known ones. I've already posted about CouchDB.

Ilya Grigorik, always a good reference, is a big fan of the more obscure (well, over here at least) Tokyo Cabinet. I'm about to try this out in my current project, so I'll post later on to say how it went.