I am interested in what is and what was the "competition" for Stack Overflow. Some of these have failed and are no longer existent, and I want to know what the notable such ones are, and why they failed, and what was the result of their policy experiments at making a good Q&A site for programming.
I am asking here since this is probably a concentration of knowledge in the area of "Q&A programming sites" and some people are no doubt aware of what else exists.
So, the question is how did key policy on allowed question types differ from SO and what were the results of these "policy experiments" for one or more of these SO competitors?