I tend to recommend more and more Netbeans that has not been proposed yet (I am blind sometimes). Netbeans is developed by Sun and support not only Java but also a few other languages (PHP,Ruby,Python, Javascript).
I use it for a while now and I am very satisfied. It is fast, provides code completion and integrates easily major JS libraries (Prototype, YUI). It has also a debugger that you can tied to FF or IE.
Try it you won't be disappointed!