It takes a bit of getting used to the interface, but to have this information publicly available is a step in the right direction.
Tip: find the area you are interested on the map. Click the pushpin on or near a feature that you are interested in. Then click the “In the area” tab. Select “Sites and Monuments Record” on the left. You will then see features nearest the pin. Click the name of the feature you’re interested in, and a new window with details pops up. From the details page you can view the feature exactly on a map, or on Google Maps. It will open endless new windows, but that’s a small price to pay for having this information freely available.
This is of course a boon for all the Stonehenge buffs out there, as you’ll be able to explore the surrounding landscape and get a better appreciation for what’s below the soil as well as above it.
[Update] The map doesn’t seem to work in Firefox or any other Mozilla-based browser (such as Flock). It does work in Internet Explorer on a PC, and Safari on a Mac, however.