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