DMDX is described as 'Win 32-based display system used in psychological laboratories around the world to measure reaction times to visual and auditory stimuli. It was programmed by Jonathan Forster at the University of Arizona'. There are three alternatives to DMDX for Mac, Windows, Linux, Android and Android Tablet. The best alternative is OpenSesame, which is both free and Open Source. Other great apps like DMDX are PsychoPy (Free, Open Source) and SuperLab (Paid).
OpenSesame is a graphical, open-source experiment builder for the social sciences. It sports a modern and intuitive user interface that allows you to build complex experiments with a minimum of effort.