Available on Download.CNet.com, 3D Shed & Shop Designer is a dedicated shed design tool which includes several preset shed models. Various computer-aided design tools, such as Blender and SketchUp, are also suitable for designing 3D models of sheds.
For a basic shed design concept, image editing software such as Microsoft Paint or Adobe Photoshop might be sufficient. General purpose CAD software packages are good options for creating complex or highly customized shed designs, although these tools often have a relatively steep learning curve. SketchUp is a free software program with an optional paid version, while Blender is completely free software.