According to Robin Schumacher, the vice president of products for DataStax, which sells a certified version of Cassandra, "A popular use case for Cassandra is time series data, which can come from devices, sensors, websites (e.g., Web logs), financial tick data, etc. The data typically comes in at a high rate of speed, can come from multiple locations at once, adds up quickly, and requires fast write capabilities as well as high-performance reads over time slices."
You can use also use MapReduce on these, so they can be good analytical stores for semi-structured data. These are highly scalable, but not usually transactional. If the relationships between the data are as important as the data itself (such as distance or path calculations), then don't use a column family/big table database.
Document databasesMany developers think document databases are the Holy Grail since they fit neatly with object-oriented programming. With high-flying vendors like 10gen (MongoDB), Couchbase, and Apache's CouchDB, this is where most of the vendor buzz is generated.
Frank Weigel from Couchbase pointed out to me that the company is moving from a key-value pair database in version 1.8 to a document database in 2.0. According to him, the "document database is a natural progression. From clustering to accessing data, document databases and key-value stores are exactly the same, except in a document database, the database understands the documents in the datastore." In other words, the values are JSON, and the elements inside the JSON document can be indexed for better querying and search.
The sweet spot for these is where you're probably already generating JSON documents. As Max Schireson, president of 10gen told me, you should consider a document database if your "data is too complex to model in a relational database. For example, a complex derivative security might be hard to store in a traditional format. Electronic health records provide another good example. If you were considering using an XML store, that's a strong sign to consider MongoDB and its use of JSON/BSON."