JavaScript editor within Eclipse
Ganymede's version of WTP includes a revamped Javascript editor that's worth a try. The key version numbers are Eclipse 3.4 and WTP 3.0. See http://live.eclipse.org/node/569
Try the Vjet Javascript IDE from ebay (installation)
Eclipse HTML Editor Plugin
I too have struggled with this totally obvious question. It seemed crazy that this wasn't an extremely easy-to-find feature with all the web development happening in Eclipse these days.
I was very turned off by Aptana because of how bloated it is, and the fact that it starts up a local web server (by default on port 8000) everytime you start Eclipse and you can't disable this functionality. Adobe's port of JSEclipse is now a 400Mb plugin, which is equally insane.
However, I just found a super-lightweight JavaScript editor called Eclipse HTML Editor Plugin, made by Amateras, which was exactly what I was looking for.
Disclaimer, I work at Aptana. I would point out there are some nice features for JS that you might not get so easily elsewhere. One is plugin-level integration of JS libraries that provide CodeAssist, samples, snippets and easy inclusion of the libraries files into your project; we provide the plugins for many of the more commonly used libraries, including YUI, jQuery, Prototype, dojo and EXT JS.
Second, we have a server-side JavaScript engine called Jaxer that not only lets you run any of your JS code on the server but adds file, database and networking functionality so that you don't have to use a scripting language but can write the entire app in JS.