Let me start with a bunch of cool shit I've found on Jaycar.
Weatherproof cigarette lighter plug. Would keep water/mud etc out.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=619
Headlight Buzzer.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=347
Driving light anti-theft kit. Farking good idea this.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=347
Electric fan (or anything) temperature controller with readout. Good for those people with electric fans who dont have motec/link engine management.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=347
Speedo corrector. I have one of these. Corrects electronic speedo's to account for larger tires/different differentials etc. Will work on 12 or 24v vehicles.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=347
12v kettle.

http://jaycar.co.nz/productView.asp?ID= ... BCATID=619
65$ 150w inverter.
http://jaycar.co.nz/productView.asp?ID= ... BCATID=680