Point your browser to YouTube EDU located at youtube.com/education. Once there, you can search videos by channel, directory or subject. Channels will give you school icons on which to click. The directory provides an alphabetical listing of schools that have posted lectures. Use any of these methods to find videos of lectures on your favorite subject.
Go to the Academic Earth website at academicearth.org. You can search by university or subject to find a lecture on video. If you're not sure what you would like to see, try "featured courses" or "featured lectures." Answer a quick survey on whether or not you think Academic Earth should offer for-credit online courses.
Also check out MIT's OpenCourseWare at the link listed below under References. This site is where MIT posts free lectures notes, exams and videos for many Massachusetts Institute of Technology courses. You can search a list of all 1,900 courses, or a list of subjects. There is also "Highlights for High School," with materials and classes useful for high school teachers and their students. MIT lectures are also available on YouTube on the MIT channel.
Another source for academic lectures is iTunes U. You can find iTunes U in the iTunes store browser. Download iTunes and open the iTunes store to access iTunes U. After you download iTunes, you will get a link for the iTunes store in the left hand windowpane. Click this link and click "iTunes U" in the upper right hand black box, and you will see "Noteworthy" lecture icons. Click on a lecture icon, or search by categories or schools. The link to a video introduction to iTunes U is located in the Resources section of this article.
Another resource to consider is the OpenCourseWare Consortium, ocwconsortium.org. The Consortium is a combined effort of over 200 higher education schools worldwide providing lectures online. Its list is arranged alphabetically by country and then by university or college. The lectures are videos, podcasts or webcasts. Each school link will bring you to that school's Consortium page where you can search by categories including courses and events.