Sun JDK 1.6
Features that I really appreciate: autocomplete, easy refactoring, incremental compiling, tooltip class help, plugins! (like for SVN), the fact that you can edit, compile, build, version, debug, and deploy from one GUI... There's more but that's what comes to mind. It definitely makes me more productive.
Caveats: You can find these features in other IDEs, of course. Also, there are some people who really prefer using CLIs and scripting everything themselves, which in some cases, results in quicker results than IDEs doing the same work --- since the scripts don't usually have a lot of overhead code. Bottom line, just experiment and find what works best for you. It's nice to have choices...