PsychoPy is one of the most popular open-source packages for running experiments in psychology, neuroscience and related behavioral sciences. Built to be: 
- Precise enough for psychophysics
- Easy enough for teaching
- Flexible enough for everything else
PsychoJS is the JavaScript port of the PsychoPy library. The idea behind PsychoJS is to make PsychoPy experiments available online, from a web page, so participants can run them on any device equipped with a web browser such as desktops, laptops, or tablets. 
Pavlovia.org is like: 
- Open Science Framework, except built with Git version control
- GitHub for psychologists, except other psychologists will find your work
- An experiment server, allowing you to run your study. Actually, it isn't a bit like that. It is exactly that!