Youtube videos

They can't be added directly but you can add an image with a link to the video like this:

<a href="http://www.youtube.com/watch?feature=player_embedded&v=YOUTUBE_VIDEO_ID_HERE
" target="_blank"><img src="http://img.youtube.com/vi/YOUTUBE_VIDEO_ID_HERE/0.jpg" 
alt="IMAGE ALT TEXT HERE" width="240" height="180" border="10" /></a>

Or, in pure Markdown, but losing the image sizing and border:

[![IMAGE ALT TEXT HERE](http://img.youtube.com/vi/YOUTUBE_VIDEO_ID_HERE/0.jpg)](http://www.youtube.com/watch?v=YOUTUBE_VIDEO_ID_HERE)