One of the grooviest directors in the history of French cinema has to Jean-Pierre Melville. I love his stylish ganster noir films from the 50's and 60's, particularly Bob le Flambeur, with the delectable Isabelle Corey, Le Deuxieme Souffle, starring Lino Ventura, Le Samouri, with a stoic Alain Delon, and many others.
Melville first came to reknown for his adaptation of Jean Cocteau's Les Enfants Terrible. He also directed L' Armee des ombres about the French Resistance against the Nazi occupation.
Recently, Criterion has been releasing a number of Melville's films on DVD. Well worth it.
Here's a groovy behind the scenes look at Le Deuxieme Souffle: