SceneMaker lets you to select a portion of a video off of YouTube, etc and share just that portion of the video without having to show the entire original video.
CoveritLive's web based software publishes your commentary in real time like an instant message. One-click publishing lets you add polls, videos, pictures and audio clips instantly, and take questions.
Screencast-O-Matic is a new web-based screencast program that is incredibly easy to use. And since it uses a Java applet, it runs on Windows, Mac, and Linux machines.