Is Netbeans free? Or is there a free (community/personal) version? What about IntelliJ - It regularly gets voted top Java IDE.
Netbeans is free with source available. Netbeans is sponsored by Sun and is available under the Sun Public License a variant of the Mozilla Public License. Eclipse is available under the Commons Public License but according to their site, its going to be converted to the Eclipse Public License.
IntelliJ is not free either in source or usage. They do have a trial version!! IntelliJ actually uses a lot of open source libs. I haven't really used it to comment on its features.
Eclipse has been built up, not only as a (superb) IDE, but also as a platform for plug-ins.
Same is true for Netbeans. Both IDE's also have a platform concept over which the IDE is built. So I can create applications over their Platforms which contain generic libraries which are used within the IDE's. This can ease lots of stuff. Netbeans also has good Plug-In capabilities. But I do not know which IDE has a better platform and plug-in design.
What's impressed me most about Eclipse is that it is written entirely in Java. I haven't had any unreasonable performance problems at all. I've used it both on Windows and Linux and the experience is absolutely seamless.
Eclipse is not as pure Java as Netbeans. This may sound a bit funny at first. In Eclipse, the Standard Widget Toolkit is used for GUI. Now SWT is not standard Java. It is possible to embed C code within Java code. So what the Eclipse group did is created a new GUI framework called SWT in which direct calls are made to the underlying OS. This bypasses the JVM. This apparently has performance benefits. But now the user needs to install the SWT libraries which contain OS dependent library files like dll's. Netbeans on the other hand uses Swing. And to run swing, just the JVM is needed. This article should make things clearer.
Also both Ant and Junit are also seamlessly integreated into the Netbeans IDE.
Personally I am not sure why Eclipse seems more popular compared to Netbeans.