Sandbox - Unreal Engine's answer to Gary's Mod?
The lead developer, Garry Newman, recently spoke to PC gamer about rumors this was going to be a follow-up to Garry's Mod. "Maybe", he replied, but the overall creative direction of Sandbox remains undecided. As it stands, the game is a mod of Unreal Engine 4, with a C# layer which will let people develop for the game without having to use C++, meaning no need to compile, and the ability to develop within the engine in effective real time.
So far the fate of the tool remains uncertain - it could even remains simply as an in-house development tool for Facepunch, but as the announcement has been made public, it seems a possible release to the community is on the cards.
Sandbox can even be ported to other engines, too. The project started life in the Source engine, before it was moved over to Unreal.
Stay tuned for further new on development from the team.