Nice writeup. My current favorite is MongoDB (I blogged a few days ago on an easy way to index and search text in MongoDB docs), but I still also use CouchDB (and covered CouchDB in my last APress book).

I have played a little with Tokyo Cabinet/Tyrant but find it a little too 'low level' - would probably be great if you only need a fast hash. Cassandra is also worth a good look: if you are a Ruby developer, the Cassandra gem can auto-install all of Cassandra and manage it for you - very elegant, really.

I admit that one reason I like MongoDB is that it is so simple to use, and the documentation is good.