Depends on personal preference. Personally, I use the MyEclipseIDE variation of Eclipse, but simple Eclipse works fine. I've seen threads about using NetBeans and other IDEs.
Unfortunately, the < 3.0 versions date back to before more "standard" IDE webapp layouts became popular. So, you may either have to import things from the source distro into a layout that make it easy for your IDE or do some IDE configuration to make it understand the JForum layout. [originally posted on jforum.net by monroe]