Hosted MongoDB and CouchDB
After a short tutorial on getting started, I am writing mostly about using both CouchDB and MongoDB as data store, either hosted yourself on EC2 (or another server external to Heroku, which is itself hosted on EC2) or commercial managed solutions like Cloudant for CouchDB and MongoHQ for a managed MongoDB service.
I like to manage my own and customer deployments on EC2 - frankly, it is fun :-)
That said, I think that there are sometimes business reasons for using hosted solutions like Heroku, Cloudant, and MongoHQ. It is a balance between development and admin costs and paying for managed platform as a service offerings.