If you love animals or are teaching a unit on animals or habitats, you will love WildEarth.tv. This is a site that streams mostly live video of animals in their natural habitats from several sites around the world. You can even chat with other viewers while watching the action.
Most notable is the live safari feed from Africa. A safari guide and cameraman in go on safari daily to look for animals. If you use Twitter and follow WildEarth (@WildEarth) they post updates when they have found interesting animals. (Please Note: This is a LIVE video feed. WildEarth does not edit the video. You will be watching live animals doing what they do in nature which may possibly include hunting and killing or mating. Use caution with young students or preview archived video to share if you are concerned about live content.)
I also enjoy the Pennsylvania woods channel. Their system of cameras have motion sensors that turn on when there is movement. If you miss a live feed, you can always view an archived video.