I'm not sure if Cassandra and Bigtable are "document" databases in the same vein as MongoDB. These guys are more like turbo-charged key-value stores - which is quite different. They trade in the ability to manipulate data for an ability to scale out linearly and have no singular point of failure (and of course various other trade-offs). Cassandra nicely gives you composite columns and column-key sorting, but that's about all you're going to get.