Add color, DB time (Rails), etc to your mongo logs. Requires just mongo gem; compatible with Mongoid 5-7, Rails, etc.

Sending emails asynchronously with mongodb

Masking production data mongodb for testing/development

A distributed lock using MongoDB as backend

Simple object mapper for MongoDB

Mongo Railtie Logging Adapter

Periodically backup new sections of the oplog for incremental backups.

Reads the oplog from a replica set and replays it to another mongo instance / rs.

mongo_persist

Mass populate MongoDB.

Rackable mongoDB based session management

Note: this gem is no longer offically supported by 10gen. For a list of MongoDB object mappers, see http://is.gd/gdehy

mongo-retry helper

Small Object Document Mapper for Mongo. you can update atomic with condition

Some tools and libraries to simplify tailing the mongod oplog

The micro ODM for Mongo. It is a full featured, repository-based ODM that doesnt get in the way.

Adds scope methods to Mongo::Collection. Inspired by Searchlogic.

Rails session stores for Mongoid, or any other ODM. Rails 4 compatible.

Accepts and serves arbitrary namespaced queue entries atomically on a FIFO basis

A simple front-end for high performance aggregate counter stores in mongodb