Coursera is one of the largest providers. They provide short courses in a huge range of topics provided by educational institutions worldwide. Try this link to see courses on education and professional development for teaching.
While Coursera is based in America, Future Learn is UK based and offers free courses from British universities. although it has a more limited range of subject areas.
The Open University offers over 700 free taster courses via Open Learn which cover a range of topics and levels and can take between 1 and 150 hours to complete. Follow this link to their teacher training resources. Their short courses may also be useful for learners who are considering university by giving them a taste of different subjects at university level.
Other providers of MOOCs are the Australian Open To Study and the American based Udacity and edX which are science and technology course providers.