MultiShow - description from Softdepia.com:
This software controls PowerPoint on a multi-display system. You can control up to three PowerPoint Slide shows simultaneously. You can also control each slide show individualy. You can also control the slide shows via an RS-232 serial interface. Each show can be run by script which will allow you to change one, two, or three of the screens at the same time. Goto http://www.iosysoft.com for more information
MultiShow is a "Free to try" program. You can download and try it for an evaluation period.