Eclipse, long the most popular Java IDE, is free and open source and is written mostly in Java, although its plugin architecture allows Eclipse to be extended in other languages. Eclipse originated in 2001 as an IBM project to replace the Smalltalk-based IBM Visual Age family of IDEs with a portable Java-based IDE. A goal of the project was to eclipse Microsoft Visual Studio, hence the name.
Java's portability helps Eclipse be cross-platform: Eclipse runs on Linux, Mac OS X, Solaris, and Windows.