At Mammatus Technology we are very excited about MongoDB. We feel MongoDB is a great way to get introduced to the NoSQL world and that there are a lot of applications where MongoDB is a great fit.Rick Hightower, one of our founders, wrote a very well received introduction about Mongo DB for InfoQ.  This is good introduction to key MongoDB concepts like autosharding, aggregation framework and replica sets.
Mammatus Technology provides a Mongo DB training course for Java Developers. 
MongoDB quickstart for Java, PHP, and Python developers
View more PowerPoint from Rick Hightower