Outside of using the T.37 on ramp/off ramp stuff offered by a number of the Cisco gateways along with the Unity fax component (free), there's nothing out there I know of that works well and doesn't cost any money - go figure.
The on ramp/off ramp is likely too heavy for demo purposes I would imagine - but you can check out more about the fax details on the IP fax service's home page here:
http://www.ciscounitytools.com/App_IPFaxConfigurationWizard.htm
