sacrificial anode ... i.e. a more reactive chunk of metal that you attach to the car. These ones sort of work... its like a half arsed version of gavanising. To make these work properly you need to put one on each panel as they don't cover a very large range. http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItem&category=50138&item=4551986720&rd=1
I had a "Rust Arrestor" brand on my Pajero and never had a spot of rust in the 8 years I owned it -
I have had the RODE electronic rust protection on my hilux 4x4 since 1991.The ERPS system is supposed to be very good as well
http://www.erps.com.au/index_erps.html
http://www.corrosion-doctors.org/Car/car-electronic-protection.htm