The Stonehenge Riverside Project Google Earth Plugin
I got the link for this from Fortean Times and it's rather good. Plenty of overlays and pics depictign the area around Stoenhenge as it was back when it was built: Seeing Beneath Stonehenge*|*Exploring the Stonehenge Riverside project with Google Earth
Just watch out for a false positive that some AV software may pop-up (Kaspersky did it in the office here).
There is another for the Avebury/Silbury Hill area as well. I'm seeing if I can track it down now.