Anyone who cares about databases absolutely must check out Datomic. If you want to know the future of relational databases for the next 15 years, I think it is here. Practically infinite client side scaling, logic based querying, ACID, runs on several foundations, supports joins, all data is permanent so it stores the entire history of every fact. You can query "right now" or in the past at any known time. The database lives on your application stack the same way git checks out your code on a development machine, and you push and pull data to bring your application up to date with other stacks.

It doesn't meet every single need you might ever have, but it sure meets a whole lot more then any other database I've seen. http://www.datomic.com