Some good free tree house designs include the Deluxe Tree House Plan from The Wood Plans Shop, the A-Frame Treehouse from Outdoor Life and the Modern Tree House Plan from Dornob. Other free tree house designs include BuildEazy's Free Tree House Plan and Ron Hazelton's Free Tree House Plan.
The Deluxe Tree House Plan features a pulley system, a ladder and bunk beds. It includes a materials and shopping list, color photos, diagrams and step-by-step building instructions.
The A-Frame Treehouse is constructed of 4x8 plywood sheets. It features a wrap-around deck and a ladder. The design allows for the house to be built in a tree or free-standing.
Ron Hazelton's plan utilizes pressure-treated wood in 2X8's and 4x4 posts to make it safe for the kids as well as simple to build.