The Armchair General offers a set of free wargames rules for 25mm / 28mm Napoleonics called A Marvelous Victory
Rules: Napoleonics
Eagles and Emperors Napoleonic Rules
Eagles and Emperors is a set of free wargames rules for the Napoleonic period based on a set of Civil War rules called The Blue, The Grey and the Butternut.
Bring Out The Old Guard Napoleonic Rules
Bring Out The Old Guard and That Vertically Challenged Mediterranean Man is a set of free wargames rules for the Napoleonic period.
eRules Napoleonics
eRules is a set of free wargames rules for the Napoleonic era. From the introduction:
eRules Napoleonic.txt is a set of rules for the Napoleonic wars designed by WargameSystems following the same principles as eRules World War II, except that Napoleonic.txt is provided free as a draft document for you to download, try out and contribute to. Let us have your suggestions for adding to it or improving it.
Eight Bells Napoleonic Paper and iPhone Rules
I wonder why this hasn’t been done before. Eight Bells is a set of paper rules, combined with an iPhone app for playing Napoleonic naval games.