I like MongoDB because it is very natural to think about databases as storing JSON-style objects. It's also useful for simple text processing pipelines. You ite

I like MongoDB because it is very natural to think about databases as storing JSON-style objects. It's also useful for simple text processing pipelines. You iterate over your database and apply the next step of preprocessing, assuming it doesn't exist in the current object (e.g. if no "POS tagged text" field exists, transform "text" into "POS tagged text"). I use MongoDB because it is C++, so it was simple to set up. CouchDB is based upon a Java stack, which I am less familiar at setting up. TokyoDB has the "tables" DB type, which I believe extends the value in "key-value" to more complicated schema-less types, but the Python support for this DB type isn't very good.