I recently went to a Google Tech Talk on MongoDB, and although the presentation wasn't very hacker oriented, I must say the setup of replication sets and sharding is ridiculously easy (http://www.mongodb.org/display/DOCS/Simple+Initial+Sharding+...). I started up a project using CouchDB and Redis, but was impressed enough with Mongo's scalability/ease of use that I think I might switch over to MongoDB for a bit.