I love watching interesting documentaries, and I just found two fantastic sites that have massive collections of documentaries that you can watch online. There are some great ones on Technology, The Future, Science, Drugs, History and Biographies etc, take a look through the list and everyone is bound to find plenty to interest them.
Best Online Documentaries
Free Documentaries.net
The ones I recommend most are: Zeitgeist, The Great Global Warming Swindle, What we still don’t know, Pump Up The Volume, The Smartest Guy in the Room.