SketchUp is a downloadable application from Google for drawing 3D pictures easily. If you have Google Earth you have seen the buildings that have been included on the maps. They were made in SketchUp.
In SketchUp you simply draw a closed, linear plane (a.k.a "a shape") and then tug on it up or down to make a three dimensional object or depression. It's that easy!
These are four different views of a park I made.
SketchUp is a lot of fun. I can see it being useful in an Art class or for Social Studies projects that involve ideal communities. Students can build them in 3D!