Unlike YouTube and Video.Google.COM, the server Videos.Yahoo.COM doesn't collect the videos. It is a search engine that finds videos spread all over the internet. This is why it can find many things that are not available at Google or YouTube. If you search for "string theory", you find a lot of lectures of various kinds, much like Brian Greene's two-minute explanation of string theory.
You may also find a lot of videos by searching for "Richard Feynman". The query "Steven Weinberg" gives you this
The woman is not necessarily the brightest person in the world - so you hear a lot of questions "so you are a physicist, right?" - "were you a physicist already several years ago?" - "and this is how you became a theoretician" and so on. ;-) But Weinberg still has some freedom to talk about cosmology. If you find some interesting videos, post them here.