Roundwood’s World has a nice post on how he modeled a marsh board for his WWI games.
No Man’s Land is a solitaire card game that’s supposed to simulate being a sniper in the Great War.
Roundwood’s World has an excellent photo illustrated post on how he built a trenches terrain board. It’s something I’ve always wanted to do.
Here’s a set of rules for Memoir ’14, a World War Variant of Memoir ’44. I rather think the great war would work better with this rules set than WWII.
This 24 page pdf has Pz8’s relatively simple free wargames rules for a variety of periods, including
1939 – 1941 Naval
1935 – 1965 Aerial
1965 – 1075 Aerial
1950 – 1975
1975 – 2010