These days I'm working a lot with Aptana IDE.
Aptana is an IDE based on Eclipse and the Studio version is free to use.
I'm using the enviroment to develop AJAX code for exporting in Adobe AIR. I have to say that I'm really impressed by it.
There are small things that are lovely in Aptana aside from the AIR plug in that is totally free and it's great.
When you create a web project in Aptana the Wizard allows you to select which AJAX Libraries that you want to import and code against. The list also exposes the Adobe Spry framework, other than the common Dojo, Ext, Scriptacolous and so on.
The Aptana project creates for you all the folder structure you need to start a new AJAX site based on one or more AJAX library.
Moreover if you upgrade to Aptana Professional (not free but cheap) you'll be able to leverage your AJAX application using Jaxer, the Aptana Ajax server, that offers a unified development model, in which the developer can
continue to use exactly the same well-known paradigms from the client —
any other server-side technologies.
I did not try it but it look very similar to Google Web Kit GWT).
Another cool thing concerning the AIR plug in is the ability to use the AIR Introspector. It's a window opened each time you launch your application from the authoring enviroment very helpful for debuggin the AIR application. In fact the AIR Introspector is something very similar to the Firebug plug in for Firefox.
The logic of the AIR Introspector is located inside the AIRIntrospector.js that you can easily open, read and extend. You can simple pass values to the Console View of the AIR Introspector suing the Console.log() method :
Another cool thing of the AIR plugin is that once installed if you go to Help->Install Adobe AIR Runtime you'll find a shortcut to install the Adobe AIR Runtime. this is something that Adobe should add to the AIR plugin for Adobe Flash cS3 and Adobe Dreamweaver Cs3 (as well as Flex Builder 3).