Several people I've talked to have, when I mentioned that I was planning on visiting Stonehenge and Avebury, remarked that Avebury is a much better site to visit than Stonehenge. And having now visited it, I can say that they are correct. Avebury is a much larger site than Stonehenge (so large in fact, that they've built a village in the middle of it). Although the stones of Avebury aren't shaped the in the way Stonehenge's are, they include some truly massive stones (bigger, I think than the largest at Stonehenge). The Avebury area is also the site of Silbury hill, a human constructed earthwork hill - no one knows why it was constructed - and West Kennet Long Barrow, an ancient burial mound. It's also fairly close to Marlborough Downs, the crop circle capital of England (and possibly the world).
So, with all this, why hasn't Avebury gained a similar amount of fame to Stonehenge? Avebury does get some tourist trade - I certainly didn't have the site to myself today - but not nearly on the level that Stonehenge gets. Now, one possible reason for this is because Avebury is a bit more remote than Stonehenge, and if you're relying on public transportation to get you there you have to pay attention to the time and the bus timetables, lest you get stranded. However, I've come up with another possible reason why Stonehenge is the more famous of the stone circles in Wiltshire:
Stonehenge is more photogenic.
Avebury is so big and so spread out (I did mention there's a village in the middle of it, right?) that it's impossible to photograph all in one go (except by air). Stonehenge, on the other hand, is compact enough that you can pretty much fit the whole of it into the frame of any average point and shoot camera. And of course, Stonehenge also looks more, for lack of a better word, engineered, than Avebury. Because the stones of Avebury aren't shaped, it's easy to discount the amount of effort that would have gone into moving them into place.
Of course, there's a plus side to this. Because Stonehenge gets all the glory, you can still visit Avebury and not be subjected to wall-to-wall tourists. So do me a favor, please. Don't tell anyone about how great Avebury is, let's just keep it our little secret. Okay?