MongoDB

MongoDB bridges the gap between key-value stores (which are fast and highly scalable) and traditional RDBMS systems (which provide rich queries and deep functionality).

I strongly feel the cloud players should adopt this approach, since so much of knowledge, best practices and standardization has gone into the design of traditional RDBMS Systems. I strongly feel ditching them is not the best way to go....

I haven't tried Mongo myself yet, would do soon....

Reference:
http://www.mongodb.org/