First World War.com - Michael Duffy, site editor - a relatively new, but Excellent comprehensive web site about the war. Constantly under construction but already full of valuable information.
The World War I document archive - Brigham Young University Library. Lots of great primary sources.
ALL QUIET ON THE WESTERN FRONT
WORLD WAR I
One of the Best World War I sites on the web.
Great site from the BBC about World War I
JOHNNY GOT HIS GUN
The Heritage of The Great War
Great Pictures & More - some pics in color - very rare