This is an amazing site that allows you to share your screen live with up to 250 viewers for free. You can even view the screencast on ipad/iPhone and Android phones. Perhaps most exciting of all is that you can use the site to control the viewed computer remotely, a useful feature for fixing any computer problems from afar. You need to download a small file to start sharing.
http://ictmagic.wikispaces.com/ICT+%26+Web+Tools
This is a great website for troubleshooting. I can have someone across campus create a share screen and take over their computer and fix it from my desk.