Subway
La Reine Margot
Amelie
Etre et Avoir
Belleville Rendezvous
Germinal
Jules et Jim
Le Train d'Enfer
Nikita
Cyrano de Bergerac
A Bout de Souffle
Les Quatre Cents Coups
Baise-Moi
Trop Belle Pour Toi
Mauvais Sang
Orphee
Belle de Jour
Betty Blue
Les Diaboliques
Le Gout des Autres
La Regle du Jeu,
He Loves Me Je Loves Me Not
La Cage aux Folles
Le Retour de Martin Guerre
Switchblade Romance
Cache is a great recent one, it's a brilliant suspense thriller.
And La Haine is a classic set in the tough suburbs of Paris. It shows you a totally different side of France to most French films
If you've seen Manon De Sources you also ought to see Jean De Florette. Both films are companion pieces, Jean being part 1 and Manon part 2.
Pretty much anything directed by Francois Truffaut or Jean-Luc Godard - many classic films from the 1960s and 70s. These two directors were at the forefront of the French Nouvelle Vague (New Wave) and brought many new ideas in film-making to the cinema screen. In general, Truffaut's films are more accessible while Godard tended to verge towarRAB surrealism and abstraction at times, creating cinema as art but sometimes difficult to watch.
As well as the very good lists above you could also try:
Belle De Jour
Delicatessen
City of Lost Children
The Green Ray (Le Rayon Vert)
The Aviator's Wife
La Pianiste
Chocolat
Rendez-Vous
Au Revoir Les Enfants
The Three Colours trilogy
Un Coeur en Hiver
Un Mauvais Fils
La Haine - what life is like in paris for some ethnic minorities, and I can say having lived there from the age of 16-18 its believable.
Le Grand Bleu - Ocean film, really good and sad too
Les Visiteurs - Very funny
As well as La Haine (one of my all time favourites) and Belle de Jour (another must see), I love Godard so would recommend from him: Tout Va Bien, 1+1, Alphaville, Weekend, 2 ou 3 Choses Que Je Sais D'elle, Masculin Feminin and Le Mepris.
In terms of more contemporary cinema The Crimson Rivers and Sur Mes Levres are good fun blockbuster type movies and Irreversible is excellent if rather disturbing.