Jutland: The Rules of the Game are a set of free wargames rules for large scale WWI Fleet Actions.
Rules: WWI
Mimi and Toutou Go Forth WWI Naval Rules
Mimi and Toutou Go Forth is a set of simple naval rules for WWI naval battles on Lake Tanganyika. Obscure, to be sure, but intriguing.
WWI Fantasy Rules
Orks and Dwarves in a WWI setting? Sure. Here’s a set of fantasy WWI Rules.
Memoir ’14 Rules
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.
Dogfight WWI Air Combat Rules
Dogfight! is a set of free wargames rules from Don Gelwwe. The rules have a bit of a different flavor from most of the genre. Don writes:
These rules are an attempt to create for players a different experience in gaming WW1 aerial combat. They are meant to confront players with broader tactical decisions that reach beyond the immediate specific-maneuver-choices of a moment within a battle to deal with what is (or will be) happening within the scope of the engagement as a whole. The demands placed on players emphasize these larger decision-making aspects of a pilot’s energies rather than the common, traditional focus on flying of most other rules. By removing the need for the player to micro-manage the piloting of the aircraft (these rules assume that the little fellow in the model already knows how to do that…), gametime can be devoted to deciding what to do rather than how. Instead of choosing which specific maneuver to use to position one’s aircraft in a particular way, players will choose amongst tactical objectives (such as “attack that enemy” or “be over there”) and then decide on the risk-level undertaken to achieve that goal. The success of an effort (attack, defense, movement, observation, etc…) is determined by dice rolls that are influenced by the pilot / aircraft quality as well as the level of risk (which reflects the difficulty of the task attempted).
You also can discuss the rules in the forums here.