Dark World War is a set of free wargames rules — actually pay what you want — for playing WWI games that include demonic and mythological creatures.
Matt Fritz has written a set of quick play rules called Day of the Dreadnaughts for recreating the Battle of Jutland. The set includes links to paper models, in case you want to try it, but don’t hvae the appropriate miniatures.
Entrenched is a set of free wargames rules for games set in the First World War.
Coal and The Kaiser is a set of free naval miniatures rules.
Author Leonard Heinz writes:
This is a game of low to moderate complexity portraying tactical naval combat in the period from 1904 through 1918. While it will not yet you play Jutland in a day, it will let you play some fairly large actions reasonably quickly. It uses a scale of 4 minutes to the game turn and 1000 yards to an inch on the playing surface. The rules focus on big ships and big guns: battleships (and particularly the Dreadnought-type battleships that began service in 1905), battlecruisers and armored cruisers.
Rudi Geudens has a page with photos of World War One guns and tanks from the Belgian Royal Museum of the Army and Military History in Brussels. Photos such as these are always useful when painting your wargames miniatures.