There are a number of good resources appearing all over the web for learning about MongoDB and Ruby. A useful selection is listed below. If you know of others, do let us know.
An introduction to MongoDB via the MongoDB shell.
In this screencast, Joon You teaches how to use the Ruby driver to build a simple Sinatra app.
For the final screencast in the series, Joon You introduces MongoMapper and Rails.
Ryan Bates' RailsCast introducing MongoDB and MongoMapper.
Ryan Bates' RailsCast introducing Mongoid.
Mike Dirolf's introduction to MongoDB at Pivotal Labs, SF.
Wynn Netherland's introduction to MongoDB with some comparisons to CouchDB.
Kyle Banker's presentation on why MongoDB is for Rubyists (and all human-oriented programmers).
John Nunemaker's articles on MongoDB and his Mongo Tips blog.
A series of articles on aggregation with MongoDB and Ruby:
An explanation of how the MongoDB drivers usually automatically support new database features.
Sync your local development db with your remote production db using capistrano.
A very simple pub/sub system.
An extensible thread safe job/message queueing system that uses MongoDB as the persistent storage engine.
A port of the Github's Resque to MongoDB.
A Rails plugin for browsing and managing MongoDB data. See the live demo.
Resource Oriented Architecture (REST) for Sinatra and MongoMapper.
A simple social news application demonstrating MongoMapper and Rails.
From Sunlight Labs, a non-trivial application using MongoMapper and Sinatra.
An example application using Mustache, MongoDB, and Sinatra.
A question and answer site similar to Stack Overflow. Live version at shapado.com.
An extension to ActiveRecord to allow the storage of arbitrary attributes in MongoDB.
ActsAsTree implementation for MongoMapper.
Machinist adapter using MongoMapper.
A delegation library for experimenting with production data without altering it. A quite useful pattern.
Testing / Matchers library using MongoMapper.
Brandon Keepers' fork of OpenIdAuthentication supporting MongoDB.
MongoTree adds parent / child relationships to MongoRecord.
A plugin for the Merb framework for supporting MongoMapper models.
A web analytics tool.
A Rack middleware component that creates HTTP endpoints for files stored in GridFS.