Minden Rose is a set of free wargames rules for playing Seven Years War battles. It is designed for 25mm or 15mm figures.
Richard Bradley’s Seven Years War rules are designed for 15mm figures on a 40mm frontage.
The Square Brigadier is a set of horse-and-musket rules based on Bob Cordery’s Chess board game inspired by Joe Morschauser.
Argad is a set of free wargames rules for skirmishes in the dark ages and for pirates in the 17th and 18th centuries.
Here’s a Horse and Musket DBA Variant — primarily for Napoleonic and Seven Years War, but since the basic principles of war in that era covered a huge number of conflicts, you could adjust as needed.