You can now record full HD quality screencasts videos (1280x720) in your browser without even creating an account at Screencast-o-matic.
You may then save the recorded movie to your desktop in Quicktime (MP4) format or directly upload the screencast to your YouTube account where it will be encoded as HD for sure - see example:
The screen capture quality is brilliant and Screen-o-matic also adds some visual hints to the screencast - the cursor size is increased and mouse clicks get highlighted with a circle - thus making it easier for people to follow the mouse in the final movie.
Screencast-o-matic, like other online screencasting apps, is a Java applet and can therefore be used on Windows, Mac or Linux machines.
Related: Make High Definition Screencasts