WeavingSimulator is fabric design software, to aid hand weavers in creating weaving patterns and immediately seeing what the fabric will look like. Supports any number of warp and weft threads, treadles, and harnesses. Runs on Windows, Linux, and OS/X.
SImple and effective, but lacking in some features. Take a look at sourceforge.net/projects/weavingsim, where I have forked it to add some new features.