One main point about Jangaroo is that it is based on a language being standardized, not inventing a new one. The main advantage is that all kinds of resources available for that language can be reused: documentation, know how, best practices, and last but not least tools.
As a Java developer, I've been using Jetbrains' IntelliJ IDEA since version 5, and I really like it. As far as I can tell, it offers the best code inspections and refactorings in the Java world.
When version 8 became final, I started using an evaluation license, but became a bit nervous how I would continue after 30 days. So I applied for an Open Source license for IDEA, and voilà, after just three days, I received a free unlimited users license for Jangaroo development!
Thank you, Jetbrains, the Jangaroo team really appreciates your support of Open Source software! I can't wait to do a screencast on how to develop, build & debug (!) a Jangaroo application using IDEA 8!
To not seem too biased ;-), in the following, I'd like to give you pointers to other IDEs that can be used for Jangaroo development.
Of course, there is the Flex Builder by Adobe. It may be the best tool to develop Flex applications, but I have to admit I never tried it for Jangaroo. As far as I know, even as an Open Source developer, after the usual trial period, you have to buy a license.
Like Flex Builder, FDT 3 is Eclipse-based and has sophisticated ActionScript 3 support, but targets at Flash (not Flex) development . I tried it with a demo license, and it works really well with Jangaroo code. Although commercial, too, you can apply for an Open Source license as well, and I think I'll do that next.
The last candidate I had a look at is the only free Open Source ActionScript 3 IDE I could find. FlashDevelop 3 (FD) is based on .net and seems to be a quite fully-featured IDE for AS2, AS3 / MXML, and HTML. Being an Open Source project, it might be the right place to add a special Jangaroo mode, although .net is not really our favorite environment (we are a Java shop...).