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.
one page
1 Page Napoleonic Skirmish
The Jackson Gamers have these free wargames rules for Napoleonic skirmishes. The best part: the rules are just one page long.
Big Battlesheet
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 Pirnt-And-Play
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 Napoleonics
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.