The X-Ray Vision-aries Blog has compiled a list of YouTube's 100 Coolest Science Experiments. I particularly enjoy their introduction to the selection:
"Although YouTube has something of a reputation as a repository for the inane mental diarrhea of stunted man-children injuring each other for cheap laughs or shrill teenage drama queens shrieking like harpies over the latest bland, interchangeable lump passing as the epitome of masculine beauty, many others thankfully take advantage of its services as a portal to share their knowledge and educate viewers.
While few of the scientific offerings formally follow the scientific method or test an explicitly stated hypothesis, even those videos veering more towards demonstrating various principles, theories, and laws still offer visitors a chance to learn something about how the world around them operates."
P.S. WonderHowTo has compiled quite a collection, too - a whole library, in fact. Check it out.