Redis (V2.4)
Best used: For rapidly changing data with a foreseeable database size (should fit mostly in memory).
For example: Stock prices. Analytics. Real-time data collection. Real-time communication.
HBase
Best used: If you're in love with BigTable. :) And when you need random, realtime read/write access to your Big Data.
For example: Facebook Messaging Database (more general example coming soon)
For other database's comments, please refer to:
http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis