TiddlySpace/tiddlyspace. And since I find TiddlyWiki interesting right now: this project is a hosting project for TiddlyWikis that can be linked to users there and can be networked with each other - sounds like an interesting platform for collaboration on projects, for example (projects that are not about code - there's GitHub and Bitbucket for that). TiddlySpace is also a direct platform where you can register, or you can simply take their open source project here and host it yourself.