JaDE is a Doom-like engine I was working on for a game I planned to make which didn't really end up happening (though I'd still like to do it one day). The game would have been something like Diablo in a Doom engine. I had originally wanted to do it just as a ZDoom mod, but quickly found it wasn't going to work unless I modified the ZDoom engine itself.
Along with the game/engine, I was also writing a blog about it, which can be found here (last updated: 2009 :P). The idea was that writing the blog would help motivate me with writing the engine+game. It's probably safe to say it didn't really work.
Part of the reason for creating SLADE 3 was to be the editor for this engine, and SLADE still lists '.jdf' as a valid file extension to open (it's just a zip, really). So in the end it did at least help with starting up my biggest project to date. As I mentioned above, I'd still like to get back to this one day, though I'd likely (you guessed it) end up rewriting it from scratch.