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.
Victory Without Quarter is a set of free wargames rules for quick play English Civil War battles.
This 28mm Life offers a large collection of free print-and-play flags from the English Civil War.
English Civil War is a modification of the Horse & Musket/Pike & Shot rules in Neil Thomas’s ‘One Hour Wargames.’
For Parliament or King is a set of free wargames rules for fighting battles of the English Civil War. The rules are designed for 28mm figures, but can be adapted to 15mm.