A lot of communities have or have had friendly competitions, whether for best question, or best answer, or questions of a particular category, etc. etc.

I think it would be beneficial to compile a list of all the different community challenges from across SE. This would have the benefit in that if later communities want to start their own challenges (totally not biased in asking this now) they can learn from historical or current challenges on other sites, see if mistakes/problems with other, similar challenges would apply on the current site and brainstorm ways to avoid them.

Could we compile such a list here?

Such competitions should be added in the answer below. They should be in the form of [Site Name](link-to-competition.com), under the appropriate header. Depending on how the site has it set up, it could be a link to the initial rule page, or a link to a search results page with a list of the relevant competitions. By doing it in this fashion, sites which want to take advantage of these statistics can easily see which sites have done that particular type of challenge before and how they did it.

Only proper challenges should be entered. One-off challenges (unless the intention was for a full series and it just never went anywhere) and site bounties should not be included.