There's a very handy web based tool called Screencast-o-matic that I've just spotted. If you go to the website and simply click on 'Create' you should be able to record and make movies of your on screen activity. This isn't anything new as there are quite a few software tools that you can either buy or download to do this, but what's good about this tool as that it all works through the web page and also allows you to create an account and save your screencasts online. This makes it much easier to share them.
The tool is very easy to use and it looks like it's even capable of recording some quite complex screen activity, such as gaming and 3D virtual worlds. You can also record audio voice over. Great stuff and all free.