The famous swim is tomorrow! Every year, on the last Wednesday in July, the feral ponies swim from Assateague Island over to Chincoteague. After a rest, some ponies are auctioned off and the rest then swim back to Assateague. The spectacle was immortalized by Marguerite Henry in her book Misty of Chincoteague. It’s a pretty popular event, as you can imagine, but you can visit the islands and view the ponies in their natural environment anytime during the year. You can learn more about the ponies at the National Chincoteague Pony Association site (warning: some pages have sound effects).
Update: here’s a news link with some photos and a video of this year’s swim.