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. With OpenSesame you can create a wide range of experiments: Psychophysics, reaction time tasks, and questionnaires, to name but a few. The plug-in framework and Python scripting allow you to incorporate external devices, such as eye trackers, response boxes, and parallel port devices, into your experiment. OpenSesame is freely available under the General Public Licence.