MongoDB Introduction

http://drupal.org/project/mongodb - a MongoDB module for Drupal

• Provides basic functionality for using MongoDB with Drupal

• Support for: Sessions, Watchdog, Block, Queue, Cache and Field storage

http://drupal.org/project/mongodb_dbtng - an experimental DBTNG driver

• Parses the SQL and translates it to MongoDB structures and queries

Drupal 8 might support MongoDB natively...