Another team seems awfully happy with Hadoop, although they've been bitten by an HDFS related bug. I can't speak too much to their experience.
Memcached of course is wonderful, and the little bit I've used Redis it has lived up to its promises.
Riak is the only non-relational database I can think of that I've only heard positive things about, but I've never personally used it.
So many of these other new data stores promise "infinite scale" and leave out the "as long as it fits in RAM" part.