It would be nice to have a compilation of high-quality conference websites, for example, in order to practice simultaneous interpretation. For this purpose I'm creating this community wiki, so please feel free to edit it instead of posting a new answer. Please post only dedicated websites that contain recorded real lectures, conferences, courses or presentations in any language (no TV or radio shows!). Please don't add written speeches, as they can easily be found on Google looking for "speeches".
Add the new conference website under the corresponding language and write a short summary on the link.
CHINESE
Phoenix TV: this is technically from TV, but it's not like a TV programme at all. It has a programme called 世纪大讲堂 Shiji Da Jiangtang, which despite a bit of scripted intro and semi-scripted Q&A at the start, is basically just one long lecture. Usually not that fast either. It's delivered to real students at a real university, but not as part of a course. The only annoying things: the subtitles in Chinese (cover them up with another window!) and the 3-4 cut-ins where an announcer suddenly summarises what's gone on, before breaking for a (skipped over) ad break.
ENGLISH
Open Yale Courses: Free and open access to a selection of introductory courses taught by distinguished teachers and scholars at Yale University
TED Conferences: TED is a nonprofit devoted to Ideas Worth Spreading. Fields: technology, entertaiment and design. You can also find more TEDxTalks here.
Academic Earth: Online courses from the world's top scholars. Many subjects.
Free Online Courses: 800+ online courses, 19,000+ videos from Top 30+ universities on 35+ categories
Khan Academy: library of over 2,800 videos covering everything from arithmetic to physics, finance, and history
MedlinePlus: this page provides links to prerecorded webcasts of surgical procedures. These are actual operations performed at medical centers in the United States. The videos last an hour. The best thing: with transcripts!
Reith lectures BBC
Oxford farming conference
Robert Reich's website: good background material and a fair number of his speech's (video)
Paul Krugman's website: good reading and video and podcasts
American Rhetoric: a very large speech bank. Many speeches come with the original audio as well as the transcript, which is very useful when training on your own.
General Assembly of the United Nations: General Debates in all UN languages, General Assembly speeches have been available for several years now, in video and audio format, complete with transcripts and their translations into English/French. Ideal for those preparing for the accreditation tests, or any other exam. Unfortunately it seems that all videos are with EN interpretation as the main sound track. Does anyone know if it's possible to access non-EN speeches in the original? Also many of the videos simply don't work - "server not found".
UN Web TV: Videos United Nations Live & On-demand
Speech Bank by Wake Forest University: links to dozens of speech websites and other compilations of political speeches (all in English).
Lectures at Berkeley: Fantastic resource of the UC Berkeley's central service for online video & audio for students and learners around the globe.
UCC Cork University lectures: Science public lecture series.
Houses of Parliament UK: Video and audio of all UK Parliament proceedings taking place in public, including debates and committee meetings of both Houses. The material is then available from an on-demand archive going back to 1st July 2009.