"Druid" costume, created to celebrate the summer solstice. As with many of the game's free costumes, this costume will only be available for a week. Europeans will want to hit the PlayStation Store tomorrow (June 18th), while North American PS3 owners will have to wait until next week, June 25th.
The long white robe should help -- as noted by Takorii -- create some excellent Assassin's Creed costumes. We'll certainly be using the beard whenever we cross a bridge in the game, screaming "You shall not pass!"