The simplest soap cutter is a stainless steel table knife. Most soapers prefer nonserrated-edge knives since they make a clean cut.
Soapmakers have adapted all kinds of tools into soap cutters. Dough scrapers borrowed from baking and putty knives and drywall tape spreaders borrowed from home improvement work very well. Even a wire cheese cutter can be put to good use for smaller bars.
Plans for a simple set of cutters for bigger bocks of soap are in Appendix C. Catherine Failor's book on transparent soap has plans for a large cutter. You can, of course, purchase cutters through soap supply companies.