BlenderNation recently came across a really interesting tool which takes 2D freehand drawings and converts them into 3D models which can be loaded into other programs such as Blender. You can literally create just about anything you can draw on the screen!
Teddy, created by Takeo Igarashi during Siggraph 1999, is the name of this Java application. It loads in a simple applet that allows easy freehand drawing. The application supports Drawing, Extrusion, Cutting, Smoothing, and Bending. It's newer revision is SmoothTeddy which includes better drawing controls and even painting. The results are quite impressive.
Teddy is a sketch-based 3D modeling software.
You can make interesting 3D models just by drawing freeform strokes.