It seems Eclipse is more popular with more people but I've been a fan of the NetBeans IDE for awhile now and I've been eagerly waiting for the release of 6.0.  This looks like an exciting release with the following features:

Java
Swing GUI Builder
Intelligent Editor
Profiler
Debugger
Updated Platform APIs

C/C++
C/C++ Projects and Templates
Source Code Editor
Multiple Configurations
Class Hierarchy Browser
File Navigation

Ruby
Ruby on Rails Support
JRuby Runtime
Code Completion
Debugger
Refactoring

Mobility
Game Builder
Device Fragmentation
SVG Graphics
Web Services
Handheld Device / Set Top Box

Web & Java EE
Visual JSF Design
Enhanced JavaScript
AJAX Enabled Components
CSS Editor
Web Services & SOA

SOA
XML Schema Editor, XSLT Designer
WSDL Designer
BPEL Designer
Service Assembly Editor
Deploy to JBI compliant runtime

Where I work we are mostly a Microsoft shop so I haven't played around with Java since I took some classes a few years ago but this release makes me really want to take up Java again.

You can download NetBeans IDE 6.0 from here.