For hobbyists this seems to be a fairly common project that can be undertaken in various ways, depending what is needed from the end product. These solutions range from 20$ to 100$ and offer varying ranges and data-rates.
There are a few potential ways for someone to go about this task. The one that was tried last year was a wireless link setup with 2 Xbee Modules. I'll also take a look into a simple solution and a more robust version depending upon the application.
Option 1: Xbee Module Wireless Serial Connection
Range: Okay (100-150ft indoor)
Bandwidth: Great (10's of kb/s)
Cost: Medium-High (Here, thats 50-100$)
Frequency: 2.4GHz.
With some advanced knowledge of the Xbee module, its possible to make a link to trip a relay remotely for about 60$ (20 each for Xbee's , and another 20 for misc. components )
Simple Schematic attaching a PIC24H to the Xbee 
Board Layout for the whole circuit was about 2.5-3sq inches.One major improvement that could be made to this board is figuring out how the Programmable IO on the Xbee's worked. You could potentially eliminate the Microcontroller altogether, which personally I think would be Ideal.
Option #2: The Walkie Talkie E-stop
Range: Short (50ft indoor)
Bandwidth: Low (none, really)
Cost: Very low (~20$)
Frequency: 27MHz, or 49MHz.
This is a pretty simple hack. Go to a store that sells kids toys. Buy a pair of toy walkie talkies (you can buy better ones and get a much better result, but this is a lot less expensive). Make sure they have the "morse code" button on them. What will be done is we'll take one walkie talkie and rip the speaker off, this will be the output of the receiver side. The transmitter side can stay stock.
Most/all of these cheap toys are AM based. Meaning that the receiver will see a change in amplitude in its band based on what the transmitter is sending. So what we do to fix all the potential noise we will get on the channel (27 and 49 MHz are popular for most child toys) is we will make a simple band pass filter at 1800 - 2200Hz. Then an envelope detector after the filter will give us an overall indication of the strength of the signal at the frequency. This could be then fed into a comparator which is triggered high when the envelope is large enough. Which can trigger whatever relay circuit you have to stop/start something remotely. With this system if you're clever you can add in multiple frequencies and get different things being triggered with moderate noise suppression.
Biggest drawback of course is the AM is directly correlated to your proximity to the car. You'll have to stay close, but for small simple projects it's price literally can't be beaten.
Which brings me to the last option that I'm trying this year for the club: Industrial AM modules. These are the ones you will find in a key-fob in a car or something similar. Low Data-rate, but massive range and very reliable.
Option #3: Linx Module(s)
Range: Huge! (Up to 1/2mile outdoor)
Bandwidth: Low(1-10kb/s)
Cost: Medium (~70$)
Frequency: 433, 415MHz.
Im trying these out right now and I'll get back to this when I know how well they work. The KH2 module I have right now is apparently slow (~250ms delay from transmit to decoded data) but for my application that's fine. Even speeding along, the car will only go ~1-2m before tripping a stop command.
I'll talk about this once it does or doesnt work. Right now, the only drawback is that you will have to etch/mill/fabricate a board for it to see any real range, and there is some -albeit minimal- RF circuitry to be done. Second drawback right now is that they can be bulky without proper tuning (They: http://www.linxtechnologies.com/ suggest a 4"x4" square for their performance tests)
With that said, hopefully in another week we'll see some progress on the 3rd option and I can tell people which I think the best method for quickly and inexpensively setting up a wireless E-stop/link is.
