Limbo, but the followup to that project, so far known only as "Project 2," is still shrouded in secrecy. Project 2, revealed a little while ago in a Danish grant report and listed as a 2014 release, is being developed in Unity, according to game director Arnt Jensen.
Limbo was constructed with its own game engine, but using a third-party engine on Project 2 will both save time, says Jensen, and allow them to release the game to "as many people as possible."
The screenshot above is the only part of the game seen thus far is above. According to Jensen, the boy may be one of several similarities between Limbo and the new game. "Many ideas were cut" from the first game, "right up until the very end," Jensen says. With Project 2, "we're still working with some of the ideas that didn't make it into Limbo."