As you all know the Vikings raided many lands. The area in Northern France which we now know as Normandy (Land of the Northmen) was heavily influenced by the Vikings settled there. One huge contribution the Normans brought with them was the developement of the Feudal System (which was developed from the Scandinavian "blood feuds". The Feudal System influenced many countries, some example being France, England and even here in Canada with the presence of Feudal Lords.

This was not the only contribution of course. Old Norse also had an influence on the French language (and of course the old Norman dialect, the closest surviving way of Norman speech of course being in French Canada)Some words of the Norman dialect: http://viking.no/e/france/norman_on_words.htm as well as influencing some modern day French.

Scandinavian contribution in Normandy: http://viking.no/e/france/contribution.html