The Solitary Wargamer offers a set of free English Civil War campaign rules called Home Fires.
Rules: Pike and Shot
Tercios Brevis Editio
Tercios Brevis Editio is a free version of the Tercios rules for the Thirty Years War.
Into Glory Ride Wargaming Rules
Into Glory Ride is a set of free wargames rules for battles set in the Renaissance and beyond. It covers the era 1500 – 1900.