Gerard De Gre and Charles Sweet were pioneering wargamers who produced a number of rules sets, including these Rules For Napoleonic Wargames, which is played on a 4 inch grid.
Rules: Napoleonics
Elan Deluxe Napoleonic Rules
Elan Deluxe is a set of free rules for playing Napoleonic wargames. Once published professionally, it now is available online. This is a very well done, and interesting set of rules. The nominal scale is 1-50, with units representing battalions and regiments.
Old Trousers 6mm
From Baccus miniatures, Old Trousers 6mm is a set of quick-play Napoleonics, played on a 2 foot x 2 foot board.
Muskets and Moustaches Napoleonic Miniatures Rules
Muskets and Moustaches (sic) is a set of miniatures rules for the Napoleonic era. The author writes:
It is a very simple game to play, all based on a handful of dice and rolls of 1-3 and 4-6. Obviously there is a bit more to it, but that is the essence!
Grand Army
In 1975, Don Lowry released a set of rules called Grand Army. Designed for 9mm !? and 15mm figures, it allows players to recreate large scale battles for Napoleonics and the American Civil War. Grand Army now is available online.