The first stable version of the new mongolite package has appeared on CRAN. Mongolite builds on jsonlite to provide a simple, high-performance MongoDB client for R, which makes storing and accessing small or large data as easy as converting it to/from JSON. The package vignette has some examples to get you started with inserting, json queries, aggregation and map-reduce. MongoDB itself is open source and installation is easy (e.g.
brew install mongodb).
If you use, or (think) you might want to use MongoDB with R, please get in touch. I am interested to hear your about your problems and use cases to make this package fit everyones needs. I will also be presenting this and related work at UseR 2015 and the annual French R Meeting.