Using Mongo with it's Java driver can be tricky
Jongo makes it easy
New white paper on Big Data and Open Source (in French) - Stefane Fermigier
SchemaCrawler is an open-source Java API that makes working with database metadata as easy as working with plain old Java objects.
SQLServer and Sybase JDBC driver - LGPL
The ultimate guide
MongoDB (from "humongous") is a scalable, high-performance, open source NoSQL database. Written in C++.
Morphia is a lightweight type-safe library for mapping Java objects to/from MongoDB