There was a Warhammer Historical game called The Great War, which used the
40k game engine. The core game covers forces for,
IIRC, 1914 and 1918 (the periods of most open, mobile warfare during the conflict) and is still available via eBay at sane prices.
There is a supplement called Over The Top which covers the middle years off the war (more trench-based, as you may expect), but the eBay prices are a little more, uh, exciting...
Highly recommended is Through the Mud and Blood by the Too Fat Lardies. The core game is pretty straightforward - you have home-made cards for each of your leaders in a deck plus some special events. Draw the card for a particular "Big Man" and he activates the units under his command, choosing from a limited number of commands (depending on his quality as a leader), to move them, direct fire or steady the troops when they come under fire themselves. The game is scaled to play from a platoon to a company, so works just fine in 28mm.
There's a lot of supplemental historical information on weapons, troops and tactics in the book, and the game plays quickly and cleanly. Of particular interest is the supplement Stout Hearts and Iron Men, which contains many scenarios from throughout the war. The first few are taken from training manuals of the time, and the book takes you through an example of play showing you the best tactics to use.
A cracking resource for Mud and Blood, and WWI gaming in general, may be found here:
http://sidneyroundwood.blogspot.co.uk/ The terrain-building articles in particular are fantastic.
