Richard Bradley offers a set of free wargames rules for World War II Skirmishes.
Sword and Dagger are a set of free wargames rules for medieval skirmishes. They’re very straightforward and at 11 pages, probably good for beer and pretzels games.
From Mammoth Games comes Planet 28, a set of free wargames rules (actually, pay what you want) for narrative science fiction games.
Planet 28 is a simple set of sci fi skirmish rules for quick narrative games between warbands of warriors, mutants, and outcasts.
Inspired by the amazing work of kitbashers and converters the world over, planet 28 is intended as a pick up and play rule system for quick games when a longer rule system isn’t viable.
Rules include –
customer character creation.
basic weapons and custom weapon creation.
a story generator for setting up and adding narrative to your games.
a campaign system that lets you build and develop your warband between games.
WWII Platoon Commander applies the mechanics of the Blitzkrieg Commander rules to a skirmish system.
Brutality is a miniatures agnostic skirmish wargame set in a science fantasy realm. You can get a free version at the link.