Do you know these projects that start for fun and just get out of control? (Ask Larry or Linus)
We're doing this software enginering class in university and our teacher gave us this nice example of bad coding called "Zuul". It's so fun just coding on it. It's some kind of text adventure engine and written in Java.I burned about two man weeks on it by now and now we got NPCs, Items and a Dialog system. All described in XML of course. It's real fun but I don't think I'm doing something usefull. Just for fun....
We GPLed it (as we rewrote 99,999% of it) and it's now a real free software project: JZuul
I'll come back to you when it has become an OS.