BlueJ is a popular Java IDE aimed at teaching the principles of object oriented programming.
BlueJ is an open source platform written in Java that's especially aimed at beginner developers. It presents items to the user in a much more easy and intuitive format than more comprehensive IDEs such as Netbeans or Eclipse, helping people get started quickly and easily whilst also understanding what's going on.
Download
The latest version of BlueJ is available from the download page, here.