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.
The Lights Are Going Out is a set of free wargames rules for WWI in 1914. The rules are designed for 15mm figures.
18 Charlie has a tutorial on making 15mm palm trees. The methods looks good, and should be quick and inexpensive.