Michael Janzen has been putting in a lot of time working on his free tiny house plans. His design is intended to be put on a trailer, much like a Tumbleweed home, but you could build it on a permanent or semi-permanent foundation. The roof lines are very innovative with a standard gable design on the ends and a shed style roof in the middle. This allows for some great window space for some nice solar gain, although with the amount of windows in the plan as it is would probably lead to too much heating for the smallness of space inside. I’d probably go with half that window area for the south side and add a generous overhang to shade the windows in the middle of the day.
And he did it all with the free version of Google Sketchup! There is a paid professional version, but Mike says the free version is just fine for the purposes of rendering drawings for a simple house like this.