Pirates anyone? Pieces of Eight is a set of free wargames rules from Andy Watkins portraying Hollywood pirate skirmishes. They are a relatively simple set of rules -- but that's a good thing! Who wants complications when you're playing pirates.
The Internet Medieval Sourcebook is a fantastic resource for miniature wargamers. I've had this one in my bookmarks for several years and haven't even begun to plumb its depths. The section on the Crusades is particularly useful.
Historical miniature gamers can never have enough research material. Here is a huge collection of primary source material on the United States from the Antebellum through Reconstruction. This University of Michigan digital library has 8,500 books and 50,000 journal articles with 19th century imprints. The collection should be of particular interest to Civil War and Mexican American war gamers.
Tanzanica is a set of free wargames rules for Darkest Africa adventures by Brett Abbott. The rules actually are a variant of Games Workshop's Mordheim miniatures rules, which are themselves free at the Games Workshop living rules archive. Mordheim is a neat set of rules for the kind of game where each player controls a small group of characters, with one uber-character and several flunkies.