Steve Burt offers a set of Napoleonic Wargames rules.
Rules: Napoleonics
Napoleonic Squares Rules
Napoleonic Squares is a set of free wargames rules for the Napoleonic Era from Steve’s Painting Shed.
Fast Play Napoleonic Rules
These Fast Play Napoleonic Rules are described by the author as DBA-esque via Fire and Fury.
Napoleonic Fury
Napoleonic Fury is a Napoleonic adaptation of the classic Fire and Fury rules.
Cold Steel Miniatures Rules for 1690 to 1850
Cold Steel is a set of free wargames rules for the period 1690 to 1950 offered by Historic Enterprises. The first release offers the basic rules, plus specific addendums for the Wars of Napoleon and the War of 1812. The nice thing about the rules is that they offer a flexible scale system, with each figure representing between 5 and 60 men. This should allow you to fight a wide variety of battles with the same rules set.