The Lord of the Rings Rules variants Yahoo Group has a set for Napoleonics.
On To Waterloo are a set of free wargames rules based on the On To Richmond American Civil War rule set.
Guard du Corps is a set of Napoleonic miniatures rules where 1 casting = 60 combatants and one turn = 15 minutes. Basing is as per Empire.
Victrix Miniatures has released a set of free wargaming rules called March of Eagles, which is designed to go with their plastic boxed miniatures. They write:
We commissioned Barry Hilton, League of Augsburg, to write a set of fast play Napoleonic rules. The rules are quick to learn, very realistic, extremely bloody and a game can easily be played within an evening. At present we are only publishing rules for infantry combat but we will be releasing supplements to cover artillery and cavalry.
These March of Eagles rules are included in each box of Victrix figures along with lots of cut out counters that are an aid to the rules. We have also included a very useful set of measuring devices within the sprue frame.