Redis. Fucking awesome database. Does exactly what it's advertised to do, with no unexpected surprises. Great documentation. Finally we can go beyond the simplistic key-value map/reduce datastores, for when you don't need all the guarantees that traditional SQL forces you to have.
Yes! I first laid hands on Redis this year while working on Notifo and it's raw speed lets me do some great things. Currently hacking on some service analytics stuff that redis is making pretty easy. Though keeping everything in memory can get expensive when we want to minimize server costs so we keep the first month of data in redis and the rest as an archive in mysql (how often is someone going to look up a notification they received more than a month ago?)
http://redis.io/
(I still use postgres and sqlite for other database needs, but I'm strongly considering moving a few of those over to redis if I have time.)