Bohica is a set of mass combat science fiction fleet rules.
rules
Sunder The Stars Starship Combat Rules
Sunder The Stars is a set of free wargames rules for Starship Combat.
Unheroic Samurai
Unheroic Samurai is a set of free wargames rules for combat with classical Samurai versus undead horrors.
La Guerre Des Ames (Souls Wars)
La Guerre Des Ames (Souls Wars) is a set of free wargames rules in which Angels and Demons fight for souls. The rules are in French, but if your browser operates the way mine does, it will automatically convert the text.
Fantasy Flyers Rules
Fantasy Flyers is a set of free miniatures wargames rules for aerial skirmishes. You can construct your own flyers
I think it would be a good alternate game if you own some of those old Man O’War flyers — the Dwarf Gyrocopter, Manticore Riders, Wyvern Riders, etc. You also could use 15mm flyers (although I can’t recall if I’ve ever seen a good variety of those), or possibly 25mm fantasy figures (but I think the distances may have to be modified for this).