See this video for a cool new human-computer interaction for 3D input on a 2D plane (pressure sensitive interactive pen display). ILoveSketch is a 3D curve sketching system that captures some of the affordances of pen and paper for professional designers, allowing them to iterate directly on concept 3D curve models.
There are plans to implement 3D surface creation, multi-touch capabilities and other tools. Your sketched 3D curve concept models can be exported into Maya ASCII format (.ma) and IGES format (.igs) for further processing.
The system which is being developed by Seok-Hyung Bae and his team from the Department of Computer Science at the University of Toronto coherently integrates existing techniques of sketch-based interaction with a number of novel and enhanced features. Novel contributions of the system include automatic view rotation to improve curve sketchability, an axis widget for sketch surface selection, and implicitly inferred changes between sketching techniques. Most commands are recognized by mouse gestures; there is hardly any visible UI. Undo and redo is by gesture. Skratch out gesture removes line segments. Intense skratch out removes entire lines.