Phil Johnson and Les Benoot offer a pdf download of fog of war rules for the horse and musket era. These are a generic add on for other horse and musket rules sets, and are intended to reflect the problems offered by visibility and scouting on command control.
Arofan Gregory offers Republic and Empire, a set of free wargames rules for 19th Century warfare.
These rules are intended to be used for any land conflict from the Napoleonic Wars through the Second Anglo-Boer War. Encompassing both colonial actiosn and conflicts between technologically advanced powers, they are intended for those who wish to recreate the lesser known conflicts of the period, for which specifically tailored rules sets are often not available. They are designed as a framework that is easily modified to fit the specific situation being recreated. Alternately, these rules are good for those who wish to play battalion-level games that move quickly, as a change of pace from more complicated games … Toward this end, any consistent basing system can be used …
Two Page 1848 – 1890 Rules is a set of free wargames rules
The Regular Army O! is a set of free wargames rules for battles of the 19th Century. It’s of a rather nebulous scale, although there is a reference to “regiments” in the rules. Mediafire rules link.
Martin Rapier offers Rifle and Kepi, a set of free wargames rules for large scale battles of the 19th Century.