A good site to remember for treehouse plans
Building a free standing deck. Build it around a tree and add railings and it's a tree fort!
Tree-house Ideas - This is a great site that provides easy to read plans for beginners. I may end up using one of their designs for the kids.
tree house 1
50 Kids Treehouse Designs. I think this is the winner for the new house :)
25 Extreme Tree Houses - Kids Activities Blog
tree house plans
How To Build A Treehouse For Kids at Ideal Home & Garden
How to Build a Treehouse (Because They're Basically the Coolest Houses Ever)
How to Build a Simple Tree House