Ed Allen’s Rencounter was the first set of free wargames rules that I found on the net. And I still think they’re one of the best.
Update: Ed Allen’s site is off the list. You can find them here.
You can also get the pages reproduced as a pdf here.
Star Trek Onitama
Onitama is a neat, two player abstract game from Arcane Wonders. Played on a 5×5 board, it is a bit like chess or checkers in that pawns move about the board capturing each other. The twist is that the movements are controlled by cards which go back and forth between the players.
This link goes to a file that offers a Star Trek skin for the game. Buy Onitama and then get the skin.
The Making of America
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 Rules
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.
Fantasy Tactics Rulebook App
Fantasy Tactics is a set of miniature skirmish rules presented as an App. You can get it on the Google Play store.