I can't even begin to list the things I have learned using sites such as youtube and teachertube. Most recently I learned how to build a raised vegetable garden using the square foot gardening method. The book about square foot gardening is a solid 300 pages, and the information was synthesized into five minute clips! I ended up reading/skimming the book regardless but the videos were extremely helpful.
In the world of education youtube tends to get a bad rap because of the content and the fact that for the most part it isn't censored. I don't think you can access pornography on the site, but vulgar language is definitely there, not to mention countless inappropriate videos that students can have access to. However, youtube has an immense amount of educational videos with the above as an example. My students have used youtube in virtually every subject/content area this year. It does wonders for visual learners - and I have a lot that prefer this learning modality. If they tell me they forget how to find area or volume, it is only a few quick clicks away, same for any topic. I find it especially helpful when they ask me something I don't know, like a definition of the water cycle. We found a pretty catchy song for this earlier this year.
I believe so much in the future and power of online videos that I spent the first two weeks of school showing my students the fundamentals of Movie Maker and giving them ample time to explore and create a video on what it means to have good character. Since then many have taken the opportunity to use videos as a means of presenting projects in other classes or creating memories of class field trips. My plan was to have each student create one informational video to share, but that just hasn't happened this year. Perhaps after standardized tests are over :)