"Radiator Saver"

http://www.golanproducts.com/trapinfo.html

http://www.jag-lovers.org/xj-s/book/CoolantFilters.html

http://www.mako.com.au/ibm_custom/search_html/TwinPack_Tefba_Radiator_Filter_Units_2.htm

They have been around for a long time, and work just great if you don't restrict coolant flow too much.