For me Dessin d'Enfants by Alexandre Grothendieck is the more concrete research work he has done . I would like to know if there are others.
 When he was teaching at Montpellier University (during the 1970's), he thaught the first year students a course on polygons (drawing, computing, ...) . The results (notes) were good , but the university establishment did not recocnize them .