I love Sir David Attenborough and his enormous collection of nature videos. Someone really needs to make some highlight reels of his work. If you haven't seen the Polar Bear vs Walrus scene from Planet Earth you don't know nature videos.
In any case I ran into David Attenborough's commentary on Darwin on Youtube. It's part of Nature Videos, which I had not heard of before. Check out the NatureVideos Youtube channel for some more David Attenborough goodness, as well as some other stuff that I haven't checked out yet.