This one is just what it says on the tin: interesting Physics topics explained in roughly a minute. With really cool animated drawings. I think the guy explains things really clearly and succinctly. See also Minute Earthfor fascinating geography.
Veritasium Derek asks members of the public about common physics misconceptions, and then explains the correct science. He also does some really exciting physics experiments, and poses questions to the audience.
Dianna Cowern provides clear and engaging explanations of everyday and not so everyday physics phenomena.
Hank Green of Vlogbrothers fame introduces interesting and unusual science concepts and ideas and keeps us up to date with the latest research news.
Brady Harran creates a large number of educational videos on several channels, including: Sixty Symbols - Physics
Smarter Every Day Join Destin as he explores and explains interesting every day science phenomena. Recent videos include the use of his top range high speed camera.
Moving onto mathematics, Vi who is a mathemusician and doodler, makes reveals just how fun and beautiful maths can be. Dont believe me? Check out her doodling in math class videos.
These three video game developers talk about import social, economic, moral and artistic issues that relate to the gaming industry. Essential watching if you are interested in the world of video games. More videos on their website. (Seriously, DO take your time to watch them all)
Here we have a very large number of mini lessons on almost every subject. Most are aimed somewhere between AS Level and first or second year undergrad, but everything is explained clearly and carefully. These videos are great for consolidating something you didn't quite grasp in lessons.