I'll keep this brief...no one needs another lecture about Stonehenge, so I'll stick to touristy photos. For more information on Stonehenge, check out English Heritage's webpage. or the University of Sheffield's excavation page. For more on Avebury, again English Heritage has a good run-through and photos. Both sites are UNESCO World Heritage Sites.
Stonehenge (duh). The site hasn't fared too well over the last few centuries. A busy road cuts nearly through the center of the site and separates the stone circle from other elements of the site and the surrounding plains. When you arrive at the site, you park, buy your ticket, then are funneled into a passageway that runs underneath the road. You pass through a cement corridor lined with large murals depicting the landscape of the prehistoric Salisbury Plain (lots of trees!), but little interpretive material is available. Then, emerging from the tunnel, you immediately see the stones themselves (and lots of sheep!). Turn around, and you'll see traffic and the visitors center.
Once at the site itself, you're essentially confined to a path ringing the stones. You can get pretty close, but no touching. From here you can appreciate the monument up close, if you're lucky enough to have a view unobstructed by picture-taking tourists.
The lack of access to the stones is not a bad idea though, considering how badly the site has been damaged by visitors over the years. The more people tread on the ground, touch the stones, or (god forbid) hack off pieces of the stones, the more irreversible damage is done to the site.
This is Avebury. It's another prehistoric site, and physically it's close to Stonehenge (about 20 miles), but as an archaeological experience it's worlds away.
The site is essentially unrestricted. You can walk along the massive (1.3 km circumference) circular earthen banks and ditches, you can touch the standing stones, and wander around the site with relatively few tourists to contend with. There's something to be said for an archaeological site like this- where a visitor is free to experience the site how he or she chooses.
There is always a conflict (at least in my mind) between the need to conserve archaeological sites and the need to make sites available to the public. While the feet and hands of the thousands of tourists who visit a site like this in a year inevitably do damage to the site's integrity, at the same time this must be balanced by the benefit of allowing the public to experience archaeological sites and the fundamental right of all people to know and experience the world's history.