Spectre Miniatures offers a set of free wargames rules for modern operations called Spectre. Click on the “rules” tab at the top of the page.
There’s also a line of 28mm miniatures that you can buy, if you don’t already have some. I’ve got moderns in 15mm already.
The Battle of Blackburn’s Ford is an introduction to the Combat Command American Civil War Rules. The pdf also includes counters if you don’t have figures.
Country Houses is a DIY card game in which players are 19th century landowners.
Dyson Logos offers Geodesic Gnomes, a free RPG:
Geodesic Gnomes is a full RPG, with background and setting, character creation and advancement rules, character sheet and even a sample adventure. In the cities of the future, people who find themselves with no place within society because of unemployment, criminal behaviour, or just mental illness find themselves pushed out of the domes that protect them from the hazards of the world around them. And since no one wants to live outside the domes, they move into the one place left for them – into the superstructure of the dome itself.
The game is a cyberpunk-styled RPG where the players take on the roles of the gnomes – the descendents in culture if not genetics of those people who moved into the superstructure of the domes. This is a world that is entirely “off the grid”. They have to scavenge for everything from food to water and in some domes even for air.
Patton’s Lorraine Campaign is a free print-and-play, hex-and-counter wargame from Lou Coatney.