Many many good offerings in here. I'll try not to name something already listed, or a big big film. Forgive me if I repeat something someone already mentioned.
I don't think I saw "Croupier" listed. Low budget British film, starring Clive Owen. I really really liked it.
"The Game", directed by David Fincher, starring Michael Douglas...Sean Penn is in it as well, in a small supporting role. Not the greatest film of all-time, but certainly interesting and visually riveting.
"Valdez is coming" Old, old film, great revenge tale.
"Sexy Beast" Watch Ghandi get very scary and intimidating.
"Excalibur" If you like Arthurian legend, this is pretty good stuff. It's pre-Braveheart/Gladiator, so the battle scenes might leave a little to be desired, but it's well done, and features small roles from a young Patrick Stewart, Gabriel Byrne and Liam Neeson.
"The Lion in Winter" Not the TNT version, but the 1968 film starring Kathryn Hepburn and Richard Harris. Also, a young Anthony Hopkins is in the film. If you like great dialogue tossed around by 2 masters, watch this film.
"Quick Change" Again, not the best comedy of all time, but funny nontheless, and even better if you're from NYC. One of the few films to feature a joust scene on bicycles.
"Ronin" Absurd plot, but, it has 2 great car chases, and DeNiro and Jean Reno running around Europe. Frankenheimer film.
Was "Swingers" listed by anyone? It should be.
"Barry Lyndon" If you are not a victim of late 20th/early 21st century television, and thus you have an attention span, check this Kubrick title out. Then watch Dr. Strangelove. Clockwork and 2001 go w/o saying, though, all of these films have their many detractors.
"All the Real Girls" Sappy at times, but, just visually mesmerizing. The sort of film that might stay with you for a bit afterwards, but, also the kind that could cause you to scoff at the romanticism of it all.
"Days of Heaven" Terrence Mallick film. Another director both revered and oft-criticized. Again, visually stunning, he loves nature and shoots it lovingly.
"Time Bandits" Terry Gilliam film, hilarious Cleese turn as a messed up Robbin Hood.
"Dark City" From the Director of The Crow, an interesting sci-fi film with William Hurt, Rufus Sewell, Kiefer Sutherland and the amazingly stunning, pre-anorexic Jennifer Connelly.
"Papillon" Dustin Hoffman and Steve McQueen try to escape a prison island.
"Marathon Man" Hoffman again, this time as a young marathon runner swept up into a crazy plot. Makes you never want to go to the dentist ever again. A great 70's film, could toss in Dog Day Afternoon, Serpico, The French Connection, etc.
That's about it, tried to mention a few genres. Not everyone is going to like everything on that list, but, it's just a sample of some of my favorites, along with many of the aforementioned films.