A quite popular mod for Half-Life 2 is called Hidden: Source, which pits humans against a lone alien who can walk on walls and the ceiling, move very fast, and see in the dark. It’s a pretty exhilirating game, and has been gaining a steady following as new beta releases come out. Beta 3 is expected to go up today at 3 p.m. EST/7 p.m. GMT and it adds new weapons, new vision modes, shader effects, new skills, and fixes many major and minor bugs.