It is one of the largest Neolithic monuments in Europe but what’s really cool about Avebury is that, unlike Stonehenge, you can walk right up to and around the stones. In fact, in a move that would probably have upset those who assembled the stone circle, the A351 road cuts straight through the middle. Just watch out for the sheep droppings. Here's their web site and here's the wikipedia article. Their webcam used to be here but doesn't seem to be working any longer. Maybe it'll come back.
PS. This may look like a duplicate of the Avebury entry on Googlesightseeing but that's because I submitted that.