Dave Winer's later creation, Frontier, has been open sourced: http://frontierkernel.org/ It works, but I wouldn't try to edit code with it. Other than its built-in language, which is actually sort of fun, but a dead-end technology now. (Still, being a language built for an outliner has interesting consequences.)
I've been working on a Frontier-descended outliner for a long time (really just the outliner, not the whole stack that Frontier represents), but up until recently, open-source toolkits couldn't do a slick outliner as the text widgets were too weak. Now I lack time to finish it, but I'm still cranking away in my spare time. (To crap something out is easy; to make a useful outliner is a harder problem that it appears.)
I've been working on a Frontier-descended outliner for a long time (really just the outliner, not the whole stack that Frontier represents), but up until recently, open-source toolkits couldn't do a slick outliner as the text widgets were too weak. Now I lack time to finish it, but I'm still cranking away in my spare time. (To crap something out is easy; to make a useful outliner is a harder problem that it appears.)