1. YouTube, I'm familiar with. Songs got me started, but I was shown clips on Physics that I could show my class; Mythbuster episodes are great science, especially when paired with Claim/Evidence/Reasoning, but DVD whole episodes mix two or three different topics, and take a whole class period; if you can find it on YouTube, you can do just one Mythbusters topic, the right topic, in 15-20 minutes.
TeacherTube was not initially very friendly, with long load times - may take getting used to.
SchoolTube was better organized, but light on content (I'll probably stick to YouTube).
KidsTube is too junior for High School upperclassmen
Discovery Education can be hit or miss on finding the right clip, and video quality stinks - pixellated!
In my experience, streaming works just fine, except during online testing weeks, and bandwidth seems to suffer after lunch, so downloading is the way to go during those times.
There are Physics video shorts on nsf.gov, and there are physics-through-sports clips available on the iPAD.
There are lots of Earth and Space Science resources at usgs.gov (US Geol. Survey) and nasa.gov. See my post on fair trade to see why .gov is such a good choice.
Instructions on how to download or embed video clips are "so easy," taking only 30 or 40 (mis-)steps each. I'm guessing that will take a. handholding, and b. practice.