If you are curious about sciences or simply want to improve your knowledge in computer science for instance, I invite to check out the following website : Academic Earth
A large scope of subjects are covered through lectures given by lecturers from the best US universities. If you do not know where to start, you should have a look at the playlists section : Academic Earth playlists
For sure, you will appreciate how organised the website is and how easy it is to find a lecture.
It is worth a visit ! Enjoy !