For rapid prototyping, I like the serverless one-file simplicity of SQLite. No need to start a db server, the app and the db can be contained in the same directory (and versioned within the same repository). You can drag it to another system and the app still works, and restarting the db from scratch is as easy as deleting the file. If needed, it's easy to manually edit with a diverse choice of GUIs. And ORMs like DataMapper make it a breeze to play with.
This is exactly the point that makes using Mongo a breeze. You can just play around with your models without having to worry about a thing. You just code it up as you go along, and your database just works.
