Aptana is still-in-early-development Eclipse-based (and therefore is written in Java) IDE for Web programmers.
It supports JavaScript, HTML, CSS syntax highlight, JavaScript syntax checking, auto-completion and even shows browser compatibility information for JS commands and CSS. You can work on levels of projects, as well as single-file editing. The IDE also supports various AJAX libraries, and you can even create projects based on AJAX libs you choose (incl. Prototype, Scriptaculous, Rico and many many more)
Absolutely, absolutely fantastic. Do, do yourself a favour and check Aptana out.
The only drawback for me is that it only supports UTF and Roman encodings, and none of Japanese-specific ones. Yeah I know in this age all sites should be created in UTF, but just.. I don't know.. can't get used to it. Being lazy I guess.