50 First Dates contains a clip from The Sixth Sense. The funny thing is it also contains a huge spoiler.

Another one I saw recently:

In America contains a clip from E.T.

War Games contains a clip from One Million Years BC

The Magdalene Sisters has a clip from the 1945 film: The Bells of St. Mary's

That's the one I was thinking of (and yes, it includes a big spoiler).

Another one is "All About My Mother" ("Todos Sobre Mi Madre), the best foreign picture winner from 2000, has scenes from “All About Eve”.

Aside from Sugar & Spice which was previously mentioned, there were a couple more recent films where scenes from Heat were used (some people were training for a heist of some kind and were watching Deniro walking on the bank counter). I'm trying to remember but it's slipping my mind.

Also the girls are watching Pretty Woman at the beginning of Romy & Michelle's High School Reunion

That's the first one that came to mind for me. Clips from:


Big Sleep, The (1946)
Bribe, The (1949)
Dark Passage (1947)
Deception (1946)
Double Indemnity (1944)
Glass Key, The (1942)
Humoresque (1946)
I Walk Alone (1948)
In a Lonely Place (1950)
Johnny Eager (1942)
Keeper of the Flame (1942)
Killers, The (1946)
Lost Weekend, The (1945)
Notorious (1946)
Out of the Past (1947)
Postman Always Rings Twice, The (1946)
Sorry, Wrong Number (1948)
Suspicion (1941)
This Gun for Hire (1942)
White Heat (1949)

Leon has a clip from singing in the rain
Amelie

The Last Dragon contains a clip from Enter the Dragon

Emanuelle in America contains a clip from the most realistically looking snuff type film i've ever seen

In kevin costners the postman the renegade army is watching and cheering sound of music (after they boo universal soldier of the screen,at least i think it was universal soldier been a while since i have seen the movie)

Fade to Black (1980) may not be that well-known, but I actually thought it was a pretty fun little horror film. The whole premise is that Eric Binford (Dennis Christopher) is a wimpy film buff who takes out his aggressions on people who've treated him like shit by becoming characters from many of his favorite films, like White Heat, Killer's Kiss, Dracula, and The Mummy. Plus, Mickey Rourke is in it playing an obnoxious jerk of a co-worker. And I don't think I've ever seen a better Marilyn Monroe look-alike than Linda Kerridge. My biggest problem was that I didn't have much sympathy for the people who got killed, and some of the dialogue between the secondary characters was stupid. But, it was really Dennis Christopher's film and I thought he did a great job. Sure the film overall could have been better, but I got it in a double feature dvd along with Hell Night for only $10, and for me it was worth it.

Big (Tom Hanks) has what I believe is French Connection in it. (when he's in the St. James hotel)

The Tingler has a scene in a silent movie theater where they are showing Tol'able David.

The Notebook (2004) features some of Li'l Abner (1940)

L.A. Confidential (1997) shows a clip of Veronica Lake in This Gun for Hire (1942)

American Beauty (1999) shows Ronald Reagan in This Is the Army (1943)

Being There (1979) features Little Caesar (1931) & Jezebel (1938)

In One Hour Photo (2002) Sy watches The Day the Earth Stood Still (1951)

I'm pretty bad w/ names & dates; had to look these up.

Can't believe no one mentioned Game of Death. Hollycow! I remember seeing that in the 80's, and how many people who watch Game of Death is not a Bruce Lee movie fan, and they had clips of his other movies. They even had paper cutouts! It was hilarious.

Does anyone know if the most recent version changed all that?

Ruby - Attack of the 50ft Woman is playing at the drive-in
Attack of the Puppet People - Amazing Colossal Man is playing at the drive-in
Play It Again Sam - Woody Allen watching Casablanca
Madhouse - at private party for Vincent Price's character they screen his latest movie which is The Haunted Palace
Blazing Saddles , well..Blazing Saddles

When discussing the decorated military history of Ed Harris' character in The Rock, there is a still shown of Harris from Borderline.

Halloween III is covered up with scenes from some sort of weird horror movie about evil Halloween masks called Season of the Witch.

All About my Mother has clips from All About Eve (when the boy is chasing after the actress for an autograph). I think it's also this movie on the TV in one of the early scenes.

If I recall Stakeout 2 has the characters watching Stakeout on TV. I believe its right at the beginning of the movie.

Back to the Future II (and III?) has scenes from Back to the Future I; Mortal Kombat Annilation- MK1; the Evil Deads have scenes from the previous movies as flashbacks (or whatever you want to call them in BttF).

Superman Returns will have unused scenes from Superman II.

Gremlins (maybe part 2?) opening was filmed for Superman (or 2?). and of course in Gremlins 1 they watch Snow White in the theater.

Back to the Future II's opening titles were from Firefox I think.

The Cable Guy has Sleepless in Seattle playing on TV.

And I know this is different, but you can see the Double Dragon arcade machine in the Double Dragon movie.

Much like Kung Pow is the Woody Allen movie, What's Up, Tigerlilly?

White Oleander has a scene where the actress character played by Renée Zellweger is watching one of her embarrassing old movies on TV and it is Texas Chainsaw Massacre 4 starring Renée Zellweger.

True Romance also has Brad Pitts character of Floyd watching Freejack on TV.

That's the one I was thinking of when I saw this thread, but I don't think it was in there just as a device... it was background to the two women (Meg Ryan's character but also her buddy Rosie O'Donnell's character) and how they got their obsession with over-the-top romantic gestures, with the "magic" they felt their romantic lives were missing...

Anyways. That's the movie I was thinking of, too.

In Donnie Darko, Donnie and Gretchen watch The Evil Dead (1982) in a theatre, you see the part when the Oldsmobile goes across the bridge. There's also a snatch of the end credits.