This is one I found just recently. It's Randy Pausch, who was a Comp Sci prof at Carnegie Melon. He's accomplished some amazing things in his life and more importantly, has led his life in a very inspiring way. Unfortunately, he was diagnosed with cancer but if you watch the video you wouldn't even know it after 5 minutes. If anything, his cancer seems to make him appreciate life more. It's long (1:15ish) but well worth it.
This is a three part series on my favorite subject, positive psychology, by Shawn Achor. You can search youtube for each part but for convenience sake I've got them all together in
this post I wrote on it. The talk is called 'The Science of Happiness and Potential' and all three parts combined comes out to about 21 minutes.