tIDE IDE
tIDE is a compact open source Java Integrated Development Enviornment. tIDE is one of the smallest IDEs, taking only about 1MB of space. tIDE leverages JDK features extensively there by having a small download size. tIDE is also very efficient and run smoothly on older machines with even less than 512MB RAM!
tIDE can integrate with external tools such as findBugs, PMD and checkstyle. tIDE editor features include auto formatting, sytax highlighting, bookmarks, auto completion, getter/setter creation etc.
NetBeans IDE
NetBeans IDE is an open source Integerated Development Enviornment supporting a wide range of languages including Java, C++ and Ruby. It is based on Java and can run on almost every platform. There are different editions available - base IDE with standard editor functionality, Java J2SE version with Swing GUI Builder and Java J2EE version which supports Web application development.
NetBeans IDE version 6.0 supports JDK 1.6. It has also extensive support for refactoring, developer collaboration, version control and profiling/debugging.
Eclipse
Eclipse is an advanced IDE written in Java. It is mainly a Java IDE, but is used with a lot of other languages. Eclipse is maintained by The Eclipse Foundation, a not for profit corporation. Many of the commercial IDEs use Eclipse as the basic engine.
Eclipse is available in multiple distributions. It is available as an IDE for Java development, as an integrated enviornment for J2EE development or as an IDE for C/C++ development.
There are plenty of open source plugins available for Eclipse. Almost any development requirement is available as a plugin, making Eclipse one of the most powerful IDEs out there.