The cool thing about redis is that it supports a bunch of different data structures: (ordered) sets, arrays, hashes, queues and more. There are a lot of db oper

The cool thing about redis is that it supports a bunch of different data structures: (ordered) sets, arrays, hashes, queues and more. There are a lot of db operations that are very natural with such support that are harder to force into the basic key/value paradigm exported by MongoDB. Also, redis is really simple to use which makes it useful for many cases where a full database might otherwise be overkill. For example, I've used it for storing results from experiments for further analysis. It's easier than a flat-file and much more performant.