I'd usually start from a problem and choose a store rather than looking at what stores excel at. So I won't speak to places where I haven't seen something solved, but here are some examples:

1. Redis is great for caching in front of a relational database, and also for running task queues. I've used it personally in both cases. Unlike MongoDB it drops data based on cache invalidation criteria, rather than dropping it randomly. Caching is one of the cases other people are proposing MongoDB for, but it's ridiculous to use MongoDB here when Redis exists.

2. Cassandra is used by Reddit and Twitter--search around and you can find lots of good writing about how they use it. Personally I've only used it indirectly via Stream.io.

RethinkDB is targeted at soft real time applications which have a need to update data across many clients. Additionally, although its a schemaless document DB, it offers functionality similar to joins/relational queries. This is a good presentation on RethinkDB: https://www.youtube.com/watch?v=Ee1v_SuECRk