I'm a big fan of MongoDB, and have used it in the past on a handful of Rails projects. It's my favorite non-SQL database for my apps running on Heroku. If you're new to Mongo and want to get started with it as a backend to your Rails apps, head over here.
Today marks the announcement of MongoDB 3.0, which looks to be a pretty hefty upgrade from previous versions. Final release should be in March. Here's the main infographic from the official announcement:
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