Tilly’s Very Bad Day is a set of free wargames rules for the Thirty Years War. They are designed as a fast play set.
The Junior General offers a set of rules — and paper ships — for the 1571 battle of Lepanto.
Pictors Studion has an English Civil War rules adaptation of Games Workshop’s Warmaster rules.
Clif Castle offers a set of free wargames rules for the pike and shot era. Clif writes:
These rules were designed with a couple of purposes in mind neither of which is to refute or downgrade any other rules set for this period. The authors of other rules sets for this era have my greatest admiration and gratitude. Dave Millward and George Gush are unquestionably the Fathers of gaming in this period. Their rules have given me 20 years of fun and history. Trevor Halsall of Newbury and Phil Barker’s DBR have added to this legacy. I hope to add to this in some small way with these rules. This period is by far the most colorful and difficult to wargame. These rules use cards and action points for movement and combat and the most popular basing system. They are designed for ease of play and, I hope, to reflect your vision of this era.
The rules use event cards, and include provisions for that most fun of the weapons of the era: the war wagon.
The Jackson Gamers have this set of free wargamesrules for the English Civil War. They use a card activation system, (not surprising, considering the acknowledged Brom influence), and d6s to resolve actions. These rules are 20 years old and in their 6th edition, so they are well-tested.