Document databases or schema-less databases are the first choice for building apps nowadays. MongoDB is a very good example of a schema-less database if one is managing their own infrastructure.
Every cloud service provider has a document database option available as a service. The advantage of these services are that you do not have to manage the instances and replications. Most of these are automated for you
I looked at a few of these services and compared them on a few usability parameters. The parameters are:
- Query Model – what sort of queries does it support - SQL and JSON are the two popular options.
- Indexes – are secondary indexes supported,
- Object sizes/Limits The biggest object you can store in it, and other limits.
The databases options are
The interesting thing about these database services is that all of them are closed source and cannot be used independent of the cloud service.
- Supports full json docmuents without any schema.
- It has a powerful SQL based query model. 
- You can manually add indexes to all fields.
- Maximum object size is 256KB. Good description of limits here.
- The service is in Beta and the total data size is limited to 10G if I understand the docs currently.
Google Cloud Datastore
- It is practically schema less and you can add fields to without specifying it anywhere.
- It has a custom JSON based query language or has an almost SQL like GQL interface.
- Automatically adds indexes for all fields in an entity.
- Developer has to define compound indexes (they call it a composite index).
- Their developer tool kit claims to suggest indexes if it encounters a query without an index. This is a unique and neat feature.
- Has been around for a while, from right when google app engine launched.
- 1MB max object size, it is the largest allowed. 20,000 indexes per collection/entity.
- A hard to understand limit on compound index size “Maximum number of bytes in composite indexes for an entity”. I think this could be made a bit clearer.
(Full disclosure => at the time of writing I work at Parse.com)
- It is schema-less but you cannot change type once you add a member in a class/collection.
- Custom JSON based query language.
- Indexing as a concept is not exposed. There are systems in the background which create compound and simple indexes for you. I think this is a great feature and reduces and reduces cognitive load.
- Supports about 300,000 apps, so it has been in production for a while.
- The objects are limited to 128KB.
- It requires defining a primary id field, apart from that it is schemaless.
- It has complicated JSON based query language. Of all the stores this is the most complicated query model.
- You can define global secondary indexes, but you have to define throughput limits while defining. This is pretty hard and complicated.
- It limits you to 5 secondary indexes.
- The object size cannot exceed 400KB.
I think this space is going to get heated up with these new services maturing, third party solutions emerging to manage them and the limits easing up. Exciting times ahead.