Here are some good ones that weren't mentioned: 

Sergeant York

In Harm's Way

Twelve O'Clock High

The Deer Hunter

Lawrence of Arabia

The Enemy Below

From Here to Eternity

The Sands of Iwo Jima

Sophie's Choice

Legends of the Fall

The Bridges at Toko-Ri

Memphis Belle

Flying Leathernecks

Flying Tigers

The Sand Pebbles

Two Women

Andersonville

The Blue Max

MASH

The Great Escape

For Whom the Bell Tolls

A Farewell to Arms

The Razor's Edge

Good Morning Viet Nam

King Rat

The Wind and the Lion

The Man Who Would Be King

Failsafe

Cold Mountain

The Beast of War

Coming Home

Catch 22

300

In the Valley of Elah

I never saw the Great Escape mentioned. Great cast, and had my fave actor growing up... Steve McQueen. Was there anybody cooler than McQueen in those days?

Mike

Steve McQueen was the essence of cool!

Sputnik listed some great older flicks. 

My Top 10 plus a couple recent gems:

Band of Brothers

Full Metal Jacket

The Deer Hunter

Saving Private Ryan

Braveheart

Hamburger Hill

Platoon

Enemy at the Gates

The Bridge on the River Kwai

In Harm’s Way

Flyboys

 Here is one starting March 2010 you can add to this list.

http://www.hbo.com/events/pacific/video.html?maven_playerId=pacific&maven_referralObject=6209624

sputnik:

Here are some good ones that weren't mentioned: 

Sergeant York

In Harm's Way

Twelve O'Clock High

The Deer Hunter

Lawrence of Arabia

OK, this one should probably be near the end of your Neflix queue as far as a war movie.  So after you've exhausted the others on this thread (excellent lists!), give this a try:

Pan's Labyrinth

Directed by Guillermo Del Toro (Hellboy movies), it's set in the aftermath of the Spanish Civil War.  1/2 war movie, 1/2 girl-and-her-imagination-movie.  It splits these two plots quite equally.  Subtitled and definately more drama than action.  Leave your mind open for fantasy (yep, fairies and creatures) and you'll enjoy.  I hope.

I hesitated a lot before posting this, as this movie may belong outside this war movie thread......

Then there's Tropic Thunder, sort of a spoof of making war movies.  Robert Downey Jr, Ben Stiller, Jack Black, to name a few of the cast.

Check out the last one on my list above - In the Valley of Elah.  It's about a father investigating the murder of his son who had just returned from Iraq.  It's one the best recent (well..2007) films I've seen.