Time
I just finished watching a great 4 hour documentary about time by Dr. Michio Kaku. Each hour explores a different concept of time: Public time, Life time, Earth time and Cosmic time. If you're a nerd like me, you can watch it for free here:
www.onlinedocumentaries4u.com/2008/08/time-documentary.html
It has only appeared on BBC so far and I don't know when it will be shown here in the US. Also, if you try to watch one of the hour clips and it gives you a message such as "This video is currently unavailable," just refresh the page and click on the clip again and it should run just fine. I am contemplating finding this miniseries on DVD if that's even out yet.
Also, you can search the rest of the website for any documentary you can think of. You can view them all for free and in their entirety. Definitely a keeper bookmark for me.