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