The "War to End All Wars" is a bit of forgotten history for most nowadays. The First World War was a brutal conflict with battles fought over feet instead of miles. It is a virtually untapped genre that modern movie makers have yet to take advantage of. Still, some of the best movies of all time were made about the First World War, such as Paths of Glory and Lawrence of Arabia.
 
Joyeux Noel (2005)
On a World War I battlefield, a momentous event changes the lives of soldiers from France, Germany, and England. This film tells the story of the extraordinary Christmas truce between warring soldiers on Christmas Eve, 1914.
Related site: Joyeux Noel Review

Gallipoli (1981)
Two Australian sprinters face the brutal realities of war when they are sent to fight in the Gallipoli campaign in Turkey during World War I. Directed by Peter Weir, and starring Mel Gibson.
Related site:
Battle At Gallipoli, 1915
The Blue Max (1966)
The tactics of a German fighter pilot offend his aristocratic comrades but win him his country's most honored medal, the Blue Max. The General finds him useful as a hero and arranges for him to test-fly an untried fighter.
Related site: WWI Aviation
Lawrence of Arabia (1962)
A biography of T.E. Lawrence. The young lieutenant Lawrence manages to get a job as an observer with Prince Feisal, the leader of an Arab tribal Army. Lawrence decides to stay and help Prince Feisal.
Related site: T. E. Lawrence
Paths of Glory (1957)
The futility and irony of war in the trenches in WWI is shown as a unit commander in the French army must deal with the mutiny of his men and a glory seeking general after part of his force falls back under fire in an impossible attack.
Related site: Teach With Movies

The First World War - The Complete Series
A comprehensive survey of the history of the First World War.

World War 1 in Color
This six-part series provides a definitive history of WWI, seen for the first time in color.
The American Doughboys: Heroes of WWI
There are not many videos on WWI, and this one is as good as it gets for a quick overview.

The Lost Battalion (2001)
Fact-based war drama about an American battalion of over 500 men which gets trapped behind enemy lines in the Argonne Forest in October 1918 France during the closing weeks of World War I.
Related Site: The Lost Battalion of WWI

All The King's Men (1999)
Feature-length drama about the mystery of Sandringham Company, which disappeared in action at Gallipoli in 1915. Directed by Julian Jarrold, and starring David Jason and Maggie Smith.
Related site: Battle At Gallipoli

All Quiet on the Western Front (1979)
A young soldier faces profound disillusionment in the soul-destroying horror of World War I. Directed by Delbert Mann, and starring Richard Thomas, Ernest Borgnine, and Donald Pleasence.
Related site: Western Front Association