The BBC has their entire collection available online. Go to Apples quicktime page there is a link on there somewhere.
Depending on what type of footage you need, try The Prelinger Files. Free to use as you please, but it's all old stuff and not of the highest resolution. If nothing else, it's good for a laugh.
A little confusing, I think. A little more research says you can reuse downloaded material as you wish, but it sounds like you you need higher quailty footage on different media, Getty charges a fee.According to the link you provided for the LOC Prelinger site, it say's 'Getty Images represents the collection for stock footage sale', so it may be free for private use but you would have no distribution rights
You can also buy at a discounted price for different resolutions (from web sized to 1080 HD)