Regiments and Roundshot is a set of very simple Napoleonics rules. Interestingly, the instructions call for using a white board for terrain.
2×2 Napoleonic 2mm Wargames Rules
2×2 is a set of free wargames rules for gaming the Napoleonic Wars with 2mm figures.
2mm is REALLY small. How does anyone paint those?
The Thin Red Line Napoleonics Rules
The Thin Red Line is a set of free wargames rules for Napoleonics warfare based on the Flames of War Rules.
A Marvelous Victory
The Armchair General offers a set of free wargames rules for 25mm / 28mm Napoleonics called A Marvelous Victory