Here is a half-completed django implementation of StackOverflow called soclone: http://code.google.com/p/soclone/. No changes since 2008 though.
Proprietary software: OK, so technically it both exists and works, but it costs money.
OSS: It has only had one man-week of labor in it. The software is not feature complete. However, if it actually worked, it would be awesome because you'd be able to browse your corporate knowledge base on your Wii. Also, it would support browsers without Javascript, because your office is cool and lets workers change things like that.
Successful open source projects -- the ones you use, the ones you love, the ones you have heard of -- are the exception. Projects like this are the rule.
FTFY. In other words, most software suck and fail, open source or not, welcome to reality.
Also, here is a full working clone of stackoverflow http://code.google.com/p/cnprog/ hosted here http://www.cnprog.com/
Any 12 year old who gets an idea for a game can put up a project on sourceforge. However, proprietary software has steeper barrier. Typically, it comes from people in an established company if not an entrenched bureaucracy. It's more interesting to choose projects that have achieved certain milestones (beyond one man-week of labor as you put it).
As well, there are plenty of proprietary projects that have failed and you have never heard of and never will. Data about failed proprietary projects is much more difficult to find. However, open source projects are almost entirely transparent about that kind of thing.