An oldie but goodie TED Talk. These are consistently the most engaging and interesting speeches. FYI I run a video site called Chime.TV and we created a channel just for TED Talks:
http://chime.tv/tedtalks
And that is the answer to the old question of "When is it a good idea to plug my site?"--when you've made something people want, in that particular situation.
Recent ones I've enjoyed are Erin McKean's Redifining The Dictionary ( http://chime.tv/tedtalks/3l9sg ) and Rives: Is 4AM The New Midnight? ( http://chime.tv/tedtalks/2gdlw )