Dino Hunt is exactly what the title says: a set of dinosaur hunting rules for 25mm – 28mm figures. Some of the dinos in that scale have to be huge.
Classic Battletech is a free 24 page set of rules for playing the original game of giant robots on hex maps.
Full Thrust Cross Dimensions is a Full Thrust rules variant. It brings together rules from the various books and tries out some new ideas.
Hostile Stars is a set of free wargames rules for fighting fleet actions in the Dark Realms universe.
All Quiet On The Martian Front is a great game set in an alternate history where HG Wells’ Martians return in 1908. It’s got a WWI vibe, with steam tanks and big cannons and motorcycles. You can take a look at the free rules here.