No sooner do I disrespect the Mozilla platform for desktop application development in my previous post, (citing Komodo as pretty much the only other known product using it), then I learn about the fact that ActiveState released it as open source.
Ars just reviewed it, touting it as a potential Eclipse for dynamic languages.
I downloaded the free trial of the commercial version years ago. Looked pretty cool but not nearly enough to justify the hefty price tag.
Will have to check it out again now that it's open...