MOOCs (Massive Open Online Course) :
Future Learn: www.futurelearn.com
Miriada X: www.miriadax.net
MIT Open Courseware: http://ocw.mit.edu/index.htm
Khan Academy: www.khanacademy.org
CodeForces: Website dedicated to competitive programming. Wide range of levels, from beginners to experts.
Jutge.org: Jutge.org is an education online programming judge where students can try to solve more than 800 graded problems using 20 different programming languages, and instructors can use it to create their own course.
Walter Lewin lectures on physics:
8.01x – Physics I: Classical Mechanics
8.02x – Physics II: Electricity and Magnetism
8.03 – Physics III: Vibrations and Waves