Courses
No Excuse List
A directory of several websites where you can study various things online.
Coursera
MOOCs (Massively Open Online Courses) run by different universities across the world. Most are completely free.
FutureLearn
Like Coursera, FutureLearn offers MOOCs run by many different universities on various topics.
Khan Academy
More free courses on a range of topics.
SkillShare
More free courses, though mainly on creative subjects.
Kriegsacademie
A Telegram channel containing a course in military tactics.
MIT OpenCourseWare
Free access to several MIT courses, with lectures, assignments and exams.