Since the last timeI posted on this thread, I've seen many more films with Miss Huppert, and all the recommendations made on the previous pages were totally worth it! What a great actress! Since she celebrated her 55th birthday this month, why not renew our appreciation of her work? 

I've looked up her filmography at Imdb and here's what I've seen (listed chronologically following Imdb's dates of release): 

La dentellière The Lacemaker (1977 / Claude Goretta) - Excellent film, great performance 
Loulou (1980 / Maurice Pialat) - A masterpiece. Great performance by Huppert. 
Sauve qui peut (la vie) Slow Motion (1980 / Jean-Luc Godard) - One of my favorite Godard films. Good performance 
Heaven's Gate (1980 / Michael Cimino) - A classic. Convincing English language performance 
The Bedroom Window (1986 / Curtis Hanson) - I don't remember much from this film anymore... too long ago 
La séparation The Separation (1994 / Christian Vincent) - Like Sara said, an underrated film with excellent performances 
Rien ne va plus The Swindle (1997 / Claude Chabrol) - Delightful performance, in a delightful film 
Saint-Cyr The King's Daughters (2000 / Patricia Mazuy) - Very interesting film with a convincing Huppert in a supporting role 
La pianiste The Paino Player (2001 / Michael Haneke) - Great film with an outstanding performance 
8 femmes 8 Women (2002 / Francois Ozon) - A lot of fun, with Huppert playing a great parody of her image as a "serious" actress 
Deux Two (2002 / Werner Schroeter) - Way over the top (film and performance), I couldn't make much of this. Need to see it again 
Le temps du loup The Time of the Wolf (2003 / Michael Haneke) - Good film with an intense performance, which I should also revisit 

Seems like I've still seen only a small part of her work in movies, so I will probably appreciate the beautiful and mysterious Isabelle even more in the future.