Eclipse is one of the most popular open source IDEs. It originated primarily from IBM's VisualAge for Java. It is a platform that allows extensibility by means of plugins (something that Maven does as well). Eclipse can be used to develop not only Java projects, but also a host of other languages by means of plugins.
As of writing this book, Eclipse 4.4.1 (Luna SR1) is the most recent edition. The screenshots in this book are for this version.