When I visited Stonehenge with my boss in 2000 we had to pay and weren't allowed near it. AFAIK we had to stay on the path surrounding the stones...
Interesting. As I've mentioned, I went in 2008 (admittedly, as part of a packaged tour), but also we were constrained to the perimeter path.
(Perhaps I "paid" as part of the tour package?)
Later I learned (from some web searches) the ability to get up-close...(if you pay and plan-ahead, for the privilege)......and as Mick West noted....show up REALLY, really early....ahead of the "normal" tour groups that are scheduled daily.
(Side-bar....en-route, our bus from London broke down, on the MotorWay!!! It was a bit humorous, in hind-sight....because I knew the thing was dead....the bus, I mean....even as it took a LONG time for anyone to admit it. I could hear and feel the darn thing.....as it died!).
This added about two-three hours to our original schedule.