The War Times Journal has a set of free Pre-Dreadnought Naval Warfare rules, designed for 1/3000 scale miniatures.
Rules: 20th Century
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.
Counterblast Adventure Battle Game
Counterblast is a set of free wargames rules for pulp-style adventures. The author writes:
Bombshell Miniatures presents the COUNTERBLAST™ Adventure Battle Game, a pulp-inspired science fiction tabletop skirmish battle game for 15mm or 28mm miniatures. COUNTERBLAST™ takes place in a dieselpunk setting of classic pulp sci-fi adventure with fast paced action and cinematic thrills, evocative of the serials that captivated the imagination of previous generations.
Wild West Shootout
From Gettysburg Gamers: a set of free wargames rules for Wild West shootouts.
Vietnam Skirmish Rules
The Black Dog of Doom offers a set of free wargames rules for Vietnam Skirmishes.
The author writes:
This is a set of wargame rules for playing a middling size skirmish set in the American/Vietnam war. The rules were written for use with 20mm metal minis and have been used in that role a number of times.
As well as the figures and terrain, you will also need to supply some standard dice, a measuring stick, lots of tiddley winks and some crunched up balls of paper!
The rules feature the use of supressing fire which will pin all but the bravest of warriors. In game terms, supressing fire reduces the ability of the supressed to interfere with your plans.
Also included are some rules for handling vehicles and artillery fire, although these features are not expected be the focus of the game. In play each player will easily be able to handle 20-30 figures.