Area 51 is a part of the Stack Exchange network where new SE sites are born. A site has to go through several stages before being launched, and having an active community is an important ingredient in any successful launch.
Some of the proposed sites are certainly interesting to many users here. What are the relevant proposed SE sites on Area 51? Please post only one site per answer and briefly justify its relevance to our community.
(The purpose of this question is to collect all relevant Area 51 proposals in one place, so they don't clutter our meta and are easy to find. Special promotion procedures are possible to make this thread more visible when needed.)