The Engine Empire is a print-and-play card game that requires only 18 cards. From the description:
In a dystopian alternate reality, four great powers vie for control of the continent. Each Empire’s soldiers lay siege to opposing cities, while their war efforts are funded by exotic exports and ingenious inventions. Only one state will orchestrate their limited resources into an Empire Engine powerful enough to lead them to victory.
It has gotten good reviews on Board Game Geek.
Gamers of a certain age will fondly remember Magic Realm, Avalon Hill’s fantasy adventure game. It is long out of print, but you can find an interesting online game facilitator here.
Multiman Publishing offers a free board game of the Battle of Epinosa (Nov. 10 – 11, 1808).
Mine Shaft Gap is a free science fiction board game. From the description:
Mine-Shaft Gap simulates a battle for resources on, above, and under the surface of a lifeless but treacherous planetoid. Players must seek out valuable metals and fuel materials and mine them, using the materials to feed their home planets, and inch closer to victory, while at the same time allocating resources to maintain or strengthen their armed forces and fortifications. Drill tanks tunnel beneath the surface of the planet while drilling stations pound away from above. Drilling stations are faster and more efficient, but are vulnerable to attack from above and even capture by enemy troops. But tunnels can cave in… And market forces are unpredictable. The resource making your planet rich today might be better off greasing the treads of your tanks tomorrow!
Postcard from the Revolution is a free wargame that fits on a postcard.