pyModSlave is a free python-based implementation of a ModBus slave application for simulation purposes. You can install the python module or use the precompiled (for Windows only) stand alone GUI (Qt based) utility (unzip and run). pyModSlave also includes a bus monitor for examining all traffic on the bus.
pyModSlave is based on modbus-tk <http://code.google.com/p/modbus-tk/ >, pySerial http://pyserial.sourceforge.net/ and pyQt4 <http://www.riverbankcomputing.co.uk >.
