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TokuMX supports most of Mongo's features besides the aggregation framework last I looked. Better locking, transaction support, better indices too -- you should check it out.

I don't know that I would call them competitors, but DB2 and Informix both have JSON support and both implement the MonogoDB protocol.

Also ToroDB (https://github.com/torodb/torodb) implements the MongoDB protocol. But it uses PostgreSQL as a backend and it's open source ;P

(ToroDB developer here)

In addition to the others already mentioned, http://hyperdex.org/ implements the MongoDB API (though I cannot vouch for all of its claims, it seems worth including for completeness).

GP may be thinking of RethinkDB.