Seconded. I looked at a lot of the NonSQL alternatives recently when writing my A/B testing framework and ended up settling on MemcacheDB simply because since Rails can already talk to memcached you get persistent, name/value storage for free (from the client's perspective).

For what it is worth, Memcachedb and Tokyo Tyrant have used the same trick. I think I came across anothe key/value store this summer that also offered memcached protocol support.