I'll start with the three obvious and most legendary ones we all know of
Snaefell Mountain Course // IoM TT 60.73 km
Nurburgring (Nordschleife + GP circuit together) 25.94 km
Circuit de la Sarthe // Le Mans 13.62 km
Eurospeedway Lausitzring has a possible 11,3km loop if all configurations are connected. However I don't think (?) anyone has actually raced on that. It has been discussed as a possible replacement for the Nurburgring 24 hours should that event someday fail to materialize due to their financial difficulties. So it's possible
Edited by SonnyViceR, 27 March 2013 - 23:41.