There used to be a Stonehenge Free Festival until the famous clash with the police that resulted in the Battle of the Beanfield. Hippies, pagans, free-spirited people against the British police in riot gear. Guess who won?
But people are allowed to gather once more at this ancient site to celebrate midsummer on June 21st and if you want to go and hang out and take part in some rituals, meet some fluffy people and sleep at the stones you can. You can expect around 20,000 others to be waiting for the sun to rise.
Check out a bit more about the Stonehenge Summer Solstice Gathering.