There have been many really creative apps iPhone/iPod touch which make it easy for an artist or designer to quickly sketch out ideas and concepts. iTracer is one of these apps. It allows you to create 3D models using basic primitives, revolutions and extrusions. While it is not as powerful as a full 3D modeling package, it is a great way to begin a model or to rapidly visualize an idea.
To begin, select the File icon located at the top left of the screen and choose the New file option. Next, select the Reflection Sample option which will automatically load a preset model and lighting scheme. Tap the Object button on the bottom to allow you to manipulate the items in the modeling window, then double tap the objects themselves. This will select the objects and allow you to delete them, leaving a blank canvas. You can also choose the Empty Scene option which will give you a blank canvas to work from. Just be sure to add a light to the scene to make it easier to view the object you are about to create. Either way will work just as well as the other.