This program (some parts of which are free) allows you to customize the math standards based activities you can assign kids. It will quiz them and assign intervention modules if needed by topic. You can track all of the kids all of the time.
TED is chock full of amazing content from the world of thinking, making, creating, innovating, etc.
Most folks know about it by now, so why include it? TED-Ed for one, but for two, it is transformative for enabling customized delivery of the content with piles of UDL features. It's just awesome.
My preferred means of collecting and sharing links.
Fully integrates with Delicious -- which had been at the top of my list until their new stacking system which I find . . . well . . . gross.
LOADS of other functionality around groups, discussions, annotations, note taking, sharing -- really well designed and gentle learning curve.