Minder Research now has a 3 channel temperature minder on the market, Receiver- Mfg Part #: MRI-200HI and the remote sensor / transmitter - Mfg Part #: RS10. That come with three different frequencies.
There are other suppliers for the wireless systems, but do not sell the extra or add on transmitters or a scanning receiver that I have found to date.
These unit would be very handy for those who are winter campers, that want to monitor there, fresh water, gray and black holding tank temperatures to help prevent them from freezing. Some times all it takes is a medium sized light bulb in the bay area to keep thing just warm enough and also not a bad idea even if one has forced hot air or heating pads under or in the areas to prevent freezing.
Its always good to know that things are working, turned on or above freezing.
The other application would be to monitor the refrigerator and freezer(s) during the hot weather to keep food from spoiling and you do not have to keep opening and closing doors to know just what the temperatures are with a quick glance.
Brett Wolfe did post some information on this issue.