How about a line voltage thermostat used for electric baseboard heaters?
You could also use a low voltage thermostat to control a contactor. (This might give you a better choice for the thermostat location.)

I know attic and gable mount exhaust fans use exactly the type of switch you are talking about. They have a knob the you use to set the temperature when to turn 'ON' and they turn off when the temperature falls about 5-15 degrees below the 'ON' temperature. Since these are ment for attic ventelation of remove excess heat, the settings are in the range of 90-150 degrees. The only difference is these switches are usually hardwired to a ventalation fan, but would obviously work going to an outlet. Can't say if that would meet code, since these switches are usually mounted in inaccessable places (i.e. attic).

Followup:

Perhaps the biggest issue (code and saftey wise) using the type of switch I'm talking about would the the rating of the switch.

If you bought a replacement thermostat for one of these fans, they are generally rated for the load of just one of these fans (something like 5 amps). Since the circuit you are likely to install this on is rated for 15 amps, and there is no way to prevent someone from plugging a 15 amp load into your outlet, I could see the thermostate being a fire hazzard because someone could easily overload it.

Since the device plugs in, why not make up a box with thermostat supplied with cord and male plug and outfeed to cord and female cord end? Plug your assembly into the wall receptacle, plug your load into the cord end. No modifications to the premises wiring and the assembly is both portable and storable.

Bus_Driver - good suggestion. Do you know where I can find a thermostat supplied with the appropriate plugs?

And now that old song, Bus Driver, once more 'round the block is stuck in my head!

bob

Thanks for the suggestions, guys! The fire hazard issue is important. I guess I could find a line voltage thermostat and hard wire my device to it. I just checked eBay - there's one that us rated at 22A 120/240 VAC. That should work with most devices, right? I'm sure I'll be pulling less that 10 amps....

I use a line voltage thermostat to turn on and off the block heater in my plane (along with a timer).

Several people have given you good ideas. However, I'm not sure why you want to cut off power to what I assume may be an outside receptacle or at least one subjected to low temps. IMO, it's not an issue. Quite the contrary, when the temp drops to a certain point, I have a freeze gurad on my pool that turns on the pump to keep the water circulating. With certain extreem conditions, electricity is not effected by temperature.

Today I was faced with a similar problem (I have a 9x10 greenhouse); wanted to control some halogen lights for heat. LMI (www.dockbubbler.com) has outlet which switches on at 35 degrees and off at 45 degrees for $29.95

For heat only, you can get a line voltage thermostat (as for electric baseboard heaters) for $20 or so and wire it up.

For cooling I use a conventional (low voltage0 thermostat connected to a neat little 24V transformer/relay combination.

HomeDespot has all of those things.

bobby c,
You're probably not as old as me but I remember it as "cab driver, once more round the block!"

A side issue is that you should have air circulating around the
greenhouse at all times. Otherwise cold spots will occur and
some things will be damaged while the stat only reads the temp
closest to the power source.

A regular 24 V thermostat, a transformer to power it and a relay, and a relay.

The relay will have both normally open and normally closed contacts.

Use the appropriate set.

How about these:
http://www.lmimg.com/Thermostatically_controlled_outlets.asp

I found the one I was looking for 35-45F for less then half the price at another site (efferslivestock dot com)
5F-T1 Thermostatically Controlled Outlet $11.49

Just search for: Temperature Controlled Outlets or Thermostatically Controlled Outlets

Here is a link that might be useful: Thermostatically Controlled Outlets