MongoDB has a nice page which explains the core MongoDB use cases: http://docs.mongodb.org/manual/use-cases/ and also here: https://www.mongodb.com/solutions

For arbitrarily structured data, document databases are great. Of course, this isn't specific to MongoDB.

Mongo is a great way to cache and distribute intermediate results in large computations, for example, if you have an efficient way to serialize and deserialize the actual in-memory representation of your data (e.g. if all your machines are the same architecture, you can just shovel blobs of memory around this way). I wouldn't use Oracle for that, the overheads are too great.

What's great specifically about MongoDB (this may apply to others, and/or not be good for your needs, but here goes)

- Easy to set up (including replication)

- Fast

- Data in "JSON" format (it's BSON internally)

- Javascript (used for map/reduce mainly)

- Good availability of libraries

Theoretically, CouchDB would have been better, but I've heard some bad stories (remember UbuntuOne)?

I don't have much experience with other NOSQL DBs (except for Redis, it's great, but I like to call it a "DB Toolkit" - I'm not dissing it, I love it)