Legio Wargames offers Legio VI Julia Augusta, a set of free wargames rules for battles of the late Roman Republic and Empire (100BC to AD250). They’re designed for 6mm, where one 20mm x 20mm stand equals a cohort.
PZ8 WWII is a set of free wargames rules for fighting battles of the Second World War using 6mm (microarmor) wargames miniatures.
PZ8 Sci Fi is a set of free wargames rules for using 6mm (microarmor) figures in a science fiction setting.
PZ8 1950-69 is a set of free wargames rules for using 6mm miniatures to refight battles of the mid to late 20th century.
Fast and Dirty 6mm Combat is a set of — well, fast and dirty 6mm science fiction combat rules. It’s just two pages long.