I recently saw a video by Neil van Niekerk titled "Just Gimme the F-Stop". It is primarily focused on the usage of on-camera flash (not the popup flash). This video is approximately 110 minutes long.http://www.youtube.com/watch?v=mivhtzg7P4s
I am new to flash related techniques and I liked this video. If you have been using flash for a while then this video may not be very useful for you.