Terence Tao @tao@mathstodon.xyz
Michael Nielsen has kindly hosted a backed up version of the Polymath wiki, which had gone offline: michaelnielsen.org/polymath/in. It is not in a form suitable for subsequent editing, but is at least a historical record of past Polymath projects. (I would imagine that any future Polymath project would likely proceed with a more modern collaboration platform, such as a Github repository.)