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.
There are definite advantages in using more than one tree for your treehouse - the treehouse can be bigger, and you have to use less bracing. The tree you see here (behind the magnolia!) has a trunk that splits into three at the base, and these trunks splay out somewhat as they grow upwards. At the height of the treehouse - about 9' (2.7 m) off the ground - one pair of trunks are touching, and the other one is about 4' (1.2 m) away. This means the design has been based on one for a closely spaced pair of trees, rather than for a group of three. The tree is a Garry oak, and they don't grow much further north than this (southern Vancouver Island), so they grow pretty slow here. A solid gnarly collection of old trunks, each about 1' in diameter at 9 ft up.
Start by figuring out how high you want the treehouse. 9' is exciting for kids but not scary. You can of course go higher, but you'll have to take more account of tree movement.