Avebury is the only place of the three linked sacred sites (avebury/stonehenge/silbury hill) that is free and accessible easily. Silbury is closed and is unstable at the moment due to past excavations. Its worth a look though if you're passing by.
http://www.crystalinks.com/avebury.html
also
http://www.mysteriousbritain.co.uk/e...bury-hill.html