Desmos is a free graphing calculator that has been available for a few years as a browser-based tool and as an iPad app. Late last month Desmos launched a free Android app.
This teacher used back-channels while his students are watching videos so that they can discuss the footage as they're seeing it. Annotating videos with Video ANT could take that process a step further by creating an archive that matches the various points in the footage.
Microsoft Mathematics provides a graphing calculator that plots in 2D and 3D, step-by-step equation solving, and useful tools to help students with math and science studies.