Warlord Games offers a set of free quick start rules for their science fiction small unit game, Beyond the Gates of Antares.
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.
Fast Play Napoleonic Rules
Jon Linney offers a set of Fast Play Napoleonic Rules. The author writes:
These rules provide a fast paced game with simple rule mechanisms that allow players to concentrate on their tactics and enjoy the ‘look’ of their tabletop battle, rather than have their heads buried in rule tables. Elements shoot and fight individually but move and test morale within their unit. One or two rule mechanisms have been ‘adapted’ from other rule systems (as I liked them); as they say imitation is the sincerest form of flattery! Players will need a table approximately 6′ by 4′ laid-out with suitable terrain. At least one D6 die and a measuring device each (measuring stick or rule), and of course two opposing armies. The measurements are for 15mm figures.
Over The Top WWI Rules
Over The Top is a set of free wargames rules for brigade level games set in the First World War.
The Rules With No Name
The Rules with No Name, the classic western wargames rules are available as a free pdf at The War Store.