Some time ago I started to use Google Sketchup for 3D modeling of tarps. It's very easy to learn and use (more specialized tasks could be complicated and may require some workarounds, but the basic ones are very straightforward). Using the measurement and dimensioning tools, it is then straightforward to transfer the design to the fabrics. Basic version (with limited pro's features like CAD import/export) is free.
Inkscape is vector drawing program working with SVG format. I use it to draw smaller patterns and so on. It has smaller set of features, but I'm sure it could replace commercial apps like Corel Draw or Adobe Illustrator for such a simple task. It's open source.