If you need to associate a slightly more complex structure (document) to a key, go for MongoDB or CouchDB. With a single query, you’re going to retrieve the whole document, and it can be a huge win. However, using them like simple key/value stores wouldn’t be as fast and as space-efficient as Membase or Kyoto Tycoon.
If you’re into geolocation, MongoDB, CouchDB and Pincaster are appropriate.
If you’re looking into horizontal scalability, go for Cassandra, Riak or HBase.