If You Tolerate This is a set of free wargames rules for fighting divisional level engagements set in the Spanish Civil War. It is designed for 15mm figures and played on a grid or hex board.
Return To The River Don is a set of free wargames rules for 15mm miniatures battles set in the Russian Civil War.
Gladius and Pilum is a set of free wargames rules for battles set in the 500BC to 500AD era. They’re designed for 15mm miniatures, with one figure equaling 50 men.
Small World Productions has a photo essay on making a nice looking 15mm village.
The League of Ordinary Gamers has a quick and easy idea for making some good looking 15mm barbed wire.