What are the use cases for non relational datastores?
Consider the situation where you have many entity types but few instances of each entity. In this case you will have many tables each with a few records so a relational approach is not suitable.
I'm looking at using CouchDB for one project and the GAE app engine datastore in the other.  For relational stuff I tend to use postgres, although I much prefer an ORM.  
Anyway, what use cases suit non relational datastores best?