One Brain Cell Vietnam Rules are a set of free wargames rules for games set in the Vietnam War. They are designed to be a simple, beer-and-pretzels set.
The Miniature Wargaming Editor
Making Modular 2D Dungeon Tiles
The Red Golem Blog has a tutorial on making modular 2D dungeon tiles.
Destroyer Commander
Destroyer Commander is a variant for Mongoose Games’ Victory At Sea that focuses on smaller ship actions. I have always wanted to do Destroyer and PT Boat games.
Resolve Napoleonic Rules
Resolve is a set of free wargames rules for Napoleonic warfare.
Simple Sails Rules
Simple Sails is a set of free wargames rules for Napoleonic fighting sail.