From left to right:
- Ruby's Playhouse
- Inspiration: Treehouse Made from Salvaged Materials
- Handmade Willow Playhouse (If you really want to, you can actually order this one, but we love that the idea started as a project for the designer's two-year-old daughter. How handy are you at weaving?)
- Backyard Playhouse at Made by Joel
- Andrew's Simply Awesome Playhouse
Small Cool Lessons: Discovering Small Space Design Ideas That Really Work
Marisa and Peter's Playfully Eclectic Home
Good Food to Share 
The Big Book of Small, Cool Spaces 
Apartment Therapy Presents 
The Eight-Step Home Cure 
Sheex Bedding
Yuck: A Fourth Grader Reveals What School Lunches Really Look Like
How to Make Fresh Pasta Dough in the Food Processor
That first "playhouse" is huge!
A couple of these are bigger than my apartment.
Those are incredible. We are building one here http://halfayard.blogspot.com/2010/06/starting-playhouse.html and it's quite a long project!