Just wanted to flag up SO-Clone, an interesting project on googleCode that wants to replicate the fundamental functionalities of Stack Overflow.
If you'd like to run a site like Stack Overflow, for example on your company's intranet for internal use by developers, keep an eye on this project, which aims to implement a Stack Overflow-like site in Django.
another project that (judging from reviews here and there) managed to clone Stack Overflow even more successfully is CNPROG
Cite this blog post:
blog Django signals
blog UML to django
blog Offline django docs