* Frequently-written, rarely read statistical data (for example, a web hit counter) should use an in-memory key/value store like Redis, or an update-in-place document store like MongoDB.
* Big Data (like weather stats or business analytics) will work best in a freeform, distributed db system like Hadoop.
* Binary assets (such as MP3s and PDFs) find a good home in a datastore that can serve directly to the user’s browser, like Amazon S3.
* Transient data (like web sessions, locks, or short-term stats) should be kept in a transient datastore like Memcache. (Traditionally we haven’t grouped memcached into the database family, but NoSQL has broadened our thinking on this subject.)
* If you need to be able to replicate your data set to multiple locations (such as syncing a music database between a web app and a mobile device), you’ll want the replication features of CouchDB.
* High availability apps, where minimizing downtime is critical, will find great utility in the automatically clustered, redundant setup of datastores like Casandra and Riak.
from this excellent DZONE article.