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I would add mongohq for a free 16mb database. It's not much, but it's enought to validate your idea if you have a small site.

MongoLab (https://mongolab.com/about/pricing/) offers 240mb for free. I've used MongoLab and MongoHQ, and both are super easy to set up.

Cool, also nice for learning mongodb if you've never used it (like me).