Bonaparte is a set of free wargames rules for Napoleonic warfare. It’s in Portuguese, so your mileage may vary.
napoleonics
Pro Patria! Napoleonic Rules
Pro Patria! is a set of free wargames rules for game set in the Napoleonic era. There are accommodations for both 15mm and 25mm/28mm games
Wargamerabbit’s Napoleonic Rules
Wargamerabbit offers a set of free wargames rules for Napoleonics that include a large selection of YouTube tutorials.
Fast Play Napoleonic Rules
Jon Linney offers a set of free Fast Play Napoleonic Rules. The author writes:
These rules provide a fast paced game with simple rule mechanisms that allow players to concentrate on their tactics and enjoy the ‘look’ of their tabletop battle, rather than have their heads buried in rule tables. Elements shoot and fight individually but move and test morale within their unit. One or two rule mechanisms have been ‘adapted’ from other rule systems (as I liked them); as they say imitation is the sincerest form of flattery! Players will need a table approximately 6′ by 4′ laid-out with suitable terrain. At least one D6 die and a measuring device each (measuring stick or rule), and of course two opposing armies. The measurements are for 15mm figures.
Fusilier Flintlock Era Rules
Fusilier is a set of free wargames rules for the the flintlock era.
The author writes:
The Flintlock Era
Fusilier is a wargames system designed for the period 1700 to 1815.The system is designed to be used with any scale of figures. There is no formal figure scale, the unit sizes being dictated by the base sizes, which in turn are based on the frontages of the prototype units. An infantry battalion might be, for example, ten 25mm figures, twenty 15mm figures or fifty 5 mm figures. The ground scale is 1 mm = 1 metre