There’s an interesting thread on Reddit detailing top miniature skirmish games. Most are obviously not free, but the threat is a good read.
Solar Scourge Science Fiction Rules
Solar Scourge is a set of free wargames rules designed for 6mm science fiction miniatures.
Corvet: Skirmish Starship Combat
Corvet is a free skirmish game of starship combat for two or more players. What makes it different is that it tries to implement a “captain’s-eye view of your ships”
Typhoon Modern Jet Combat
Typhoon is a set of free wargames rules for modern jet combat. From the author:
Typhoon is an introductory set of modern jet combat rules that does not require hex mats or any special flight stands for miniatures. It is designed as a quick playing, low complexity game for gamers who are interested in flying multiple aircraft with aircraft from the late 70s up to the Eurofighter and F-35.
Free From Amazon: Hardtack and Coffee
Amazon offers a free Kindle Edition of Hardtack and Coffee.
Hardtack and Coffee or The Unwritten Story of Army Life (1887) is a memoir by John D. Billings. Billings was a veteran of the 10th Massachusetts Volunteer Light Artillery Battery in the American Civil War. Originally published in 1888, Hard Tack and Coffee quickly became a best seller, and is now considered one of the most important books written by a Civil War veteran. “Hard Tack and Coffee” is not about battles, but rather about how the common Union soldiers of the Civil War lived in camp and on the march. What would otherwise be a mundane subject is enlivened by Billings’ humorous prose.