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.
The Lead Gardens Blog offers a set of iniative cards for English Civil War games. They’re designed to work with the Victory Without Quarter rules, but no doubt could be used for many others.
Mitch’s Wargaming and Model Making Blog has a short illustrated article on making an English Civil War style house. I’ve used similar techniques to build a town of houses for my Gothic Horror games.
Baccus 6mm has a good painting guide for the English Civil War. There’s also information on flags.
The Oliver Cromwell Society has a website with basic information on the English Civil War. In light of the growing popularity of Warhammer Historicals’ English Civil War miniature wargaming rules set, this could be useful in filling in some gaps players’ knowledge.