Redis  - I wrote about why in October ( <a href="http://simonwillison.net/2009/Oct/22/redis/" rel="nofollow">http://simonwillison.net/2009/Oct/22/redis/</a> ) and it's gained some awesome new features since then, including ordered sets and blocking fetch against queues. It's a very different beast from the other stuff that gets branded as NoSQL.