This is a simple immobiliser based on a PIC12F629 and an ID-12 chip from innovations. This can be built for about $50.
The ID-12 chip is/can be remotely mounted away from the main PCB, behind a panel with no external components viewable.
If someone carjacks you or something like that (after you have started the car) then if they stop the car, it will not be startable again.
Cheap and effective
I apologise for the slight lack in detail. This was a project i done some time ago and it has been a long time since i touched it. I have posted this on here as it was requested by a few people. I will be happy to answer questions though.
Step 1: The Schematic and how it works
Initial setup is done by shorting the jumper and then reading up to 10 tags in sequence. This overwrites each tag if it's alreading in eeprom.
I know the schematic is hard to read, click on the 'i' in the top left then click on the 'original file' link for an uncompressed version...