MongoDB, and CouchDB. Both very similar, very different databases. MongoDB is fast (they say it's almost as fast as writing to /dev/null), has cool update featu

MongoDB, and CouchDB. Both very similar, very different databases. MongoDB is fast (they say it's almost as fast as writing to /dev/null), has cool update features (i.e. increment count by 4, if count is less than 10), and great documentation, but it's dangerously unreliable by default. CouchDB is rock solid, and has a very nice map-reduce setup (allowing your aggregations to update themselves quite quickly as underlying data changes), but the documentation is very bare, and it's not very fast (somewhere between 50 updates per second, to 3000 updates per second, depending on how you do things).