This particular design requires two or three trees (or branches) in close proximity. It was made over the course of several weekends using new, pressure-treated wood for the support structure and floor and an old fence was recycled for the sides. The roof is a camouflage-pattern tarp. It's not weather-proof, but it stays pretty dry inside: a three-season treehouse, but best for summer! It was made with my 4, 6 and 8-year old children in mind, but has been a hit with visitors of all ages.
Note: this treehouse was built in summer 2009, taken down due to tree growth in fall 2013, and rebuilt in spring 2014. I've updated the text to reflect the (minor) changes I made, but there are a mix of old and new photos throughout.