The Solitary Wargamer offers a set of free English Civil War campaign rules called Home Fires.
pike and shot
Tercios Brevis Editio
Tercios Brevis Editio is a free version of the Tercios rules for the Thirty Years War.
English Civil War – One Hour Wargame
English Civil War is a modification of the Horse & Musket/Pike & Shot rules in Neil Thomas’s ‘One Hour Wargames.’
Great Northern War – One Hour Wargames
Alan Saunders offers Great Northern War, a set of free wargames rules that he describes as:
A heavily modified version of the Horse & Musket/Pike & Shot rules in Neil Thomas’s ‘One Hour Wargames’ specifically for use with the Great Northern War.
Japanese Clan Wars Rules
An Honorable Victory is a set of free wargames rules for battles set in Japan’s 15th and 16th century clan wars. The author writes:
An Honourable Victory! are an abstract (but not too abstract) set of rules designed for the table-top wargaming of the Clan Wars of Japan during the 15th and 16th centuries.
After one or two games you’ll have them down pat.