Most Irregular is horse and musket rules extension to the Fast and Dirty skirmish rules, found on the same page.
skirmish
Malevolence Sci Fi Skirmish rules
Inspired by Hasslefree’s miniatures, Malevolence is a small-scale sci fi skirmish game. Better yet — the author says that he uses it for his Zombie Games.
Dying Star Oblivion Skirmish Quickstart Rules
Dying Star Oblivion is an skirmish game based on Samsun Lobe’s Dying Star series
. You can get the files via their Facebook page here.
Colonial Skirmish Rules
The Tyneside Wargames Club offers their free wargames rules for Colonial skirmishes.
Across The Dead Earth Post Apocalyptic Skirmish Rules
Across The Dead Earth is a commercial product, but for now, the company offers a free pdf version.