The Joint-terraining Exercise is an initiative that I've been hosting sporadically since quite some time ago. It starts off with a near-empty landscape and everybody who wants to is free to join in and fill the map up. While it might not be typically playable, this terrain can be considered an asset for anyone to be used freely - the terrain mostly suits itself for some kind of RPG-style map. It is not completely walkable, and anybody who wishes to make use of it should have some editing and pathing work on their hands to complete it. The original thread for it can be found here.
Should you feel like using this terrain, there is no need to mention all the authors; if you wish to credit, credit Sc2Mapster. Just for the record though, here is a list of all users who have worked on the terrain in this map, sorted alphabetically:
Miscellaneous credits go to Pshyched for his "evolution" video of the map.