One amazing resource that's often overlooked is the Prelinger Archives, which is available at the Internet Archive . Everything in the Archive is in the public domain, meaning it can be used for any purpose whatsoever - even commercial use - with no permission or licence fee needed. You can download the clips in a variety of formats, such as MP4, MPEG-2 or Ogg video files.
Of course, the biggest problem is finding suitable material. Luckily, there is a decent search feature, but even this is not totally reliable. Many of the home movies are untitled and have not been indexed, so it's sometimes pot luck what they contain. I found that using the search box at the top of the home page gave better results than using the keyword search.
One very useful feature is the thumbnails - if you open a clip's page, on the top left is a link to thumbnail images, which are automatically created every 30 seconds or so for the duration of the clip. This is very useful for getting a rapid idea of what's in the clip.
The range of content in the Archive is amazing. There are full-length colour documentaries, music clips, silent movies, home movies, educational films and all manner of more weird stuff from most periods of the 20th Century. It's an incredible resource, not just historically, but also for those looking for interesting video or sound clips to spice up a home movie or business presentation.
As an example of what can be done with it, here's a 5-minute music video I recently made using only material from the Prelinger Archives (the music is a reworking of one of my own songs by the talented Barry Snaith). It took about 2 weeks to make in my spare time, and most of the time was taken finding the clips.
I used about 16 separate videos from the Archives, mostly home movies and popular music clips. The software I used was Avid Studio (which is now Pinnacle Studio). This has a great collection of professional effects from Red Giant (now only included in the Ultimate version of Pinnacle Studio). Although Avid Studio hasn't had updates for a long time, it does work perfectly well in Windows 8.