Today MongoDB released a free tier for MongoDB Atlas. Atlas is a database-as-a-service run by MongoDB itself.
The free tier is their Atlas M0 instance and contains 512MB of storage.
Along with the free tier announcement, they also debuted a tool called MongoMirror which will facilitate migration of existing replica sets into MongoDB Atlas.
One nice contrast of Atlas vs. Amazon or Google's cloud database offerings is that they are planning support for Azure and Google Cloud Platform as well which will offer cross-vendor redundancy.
https://www.mongodb.com/blog/post/announcing-free-tier-and-live-migration-tool-for-mongodb-atlas
https://www.mongodb.com/cloud/atlas