Andy Watkins has one page set of rules for games involving Roman gladiators. You can use the fine Wargames Foundry 28mm Gladiators to fill your arena.
The Jackson Gamers have these free wargames rules for Napoleonic skirmishes. The best part: the rules are just one page long.
Big Battlesheet is a set of free wargames rules for grand tactical 20th century combat. The rules fit on one page. A company is composed of 3 elements.
Big Battlesheet Direct Download Word File
Postcard Empire is a free print-and-play boardgame for 2 – 4 players. As the name suggests, the game board fits on a postcard, while the rules fit on the back. The black-and-white pdf is free. The site also sells a very inexpensive color postcard.
Shako and Drum is a set of free wargames rules for the Napoleonic wars. It’s a one-page set of rules, for those who like that sort of thing.