Big Battle Napoleonics is a set of free wargames rules by David Wareham.
Rules: Napoleonics
Bonaparte Napoleonic Rules
Bonaparte is a set of free wargames rules for Napoleonic warfare. It’s in Portuguese, so your mileage may vary.
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.