I know I’m late to the game, but I was looking over Mozilla Bespin — a web-based IDE with a surprising amount of features, including source control and syntax highlighting. Right now, it’s all about web techs, with a strong focus on HTML, Javascript, and CSS, but it is extensible and seems like it is headed in the right direction.
This could be a cool way to develop/test the GamesByEmail game I’ve been fiddling with, perhaps. I don’t think code-completion is in there, yet, which will ultimately be a must-have, but I can play for now…
Big bonus: the editor is high-performance, and is implemented with my favorite, the HTML5 Canvas tag. See also my experiments right here (I’d recommend planetaryZ2, and don’t forget to mouseover/click a planet).
There is some work going on integrating eclipse with this… not sure exactly what, yet, though. See Eclipse in the Cloud and E4/Bespin.