YouTube has launched a new video editor. It allows you to trim videos that you find on YouTube or mix together videos that you have uploaded to YouTube. This could be very useful if you find a video on the site that you want to use in class but you only require particular segments of it. You may also find multiple videos that you would like to merge together as one and then download.
Richard Byrne, a fellow blogger that I follow, has created a very simple tutorial that explains how to use it. You can access that tutorial here http://docs.google.com/present/view?id=df6bwk2v_552gj98kwf9