TED, which stands for Technology, Education and Design, though the scope has broadened out over the years, is undoubtedly the most significant site of its kind. It hosts a multitude of categorized videoed presentations by respected people from all kinds of fields covering a host of topics. TED has the taglines “Ideas worth spreading” and “Riveting talks by remarkable people, free to the world“. If you’ve never seen it, I thoroughly recommend spending some time investigating it in much more detail. I’ve been using it for some years.
FORA.tv is a similar site to TED. It too has videos of presentations, and discussion, given by noted people. Its tagline is “Fuel the Enlightenment“. I was rather pleased when I stumbled upon it. Also worth spending some time browsing the content.