Various ones from nature settings, clouds, oceanography, stunts, playgrounds, backgrounds...the list is endless. Just seems to me that they have photo sites for yearly membership so why not have video?
The videotraxx libraries from digitaljuice are pretty useful.. and relatively cheap. I have videotraxx 1 and they have released version 2 - with version 3 out soon. Each library has around 3000+ clips in many different categories. They go for about $500 I think (may be cheaper now).