This paper describes an open-source software for real-time rhythm annotation. The software integrates several modules for graphical user interface, user management across a network, tap recording, audio playing, midi interfacing and threading. It is a powerful tool for conducting listening tests, but can also be used for beat annotation of music or in a game setup. The parameters of this software, including the real-time constraints, are not pre-defined in the code but can be easily changed in a settings file. Finally, the framework used allows for scalability, as it was developed in openFrameworks. We show the usefulness of the software by applying it in a cross-cultural beat tapping experiment during the ISMIR 2012 conference. An analysis of the collected real-time annotations indicates that listeners encounter difficulties in synchronizing to music in presence of unfamiliar rhythmic structures and instrumental timbres.