Mutant Hunter Assassin

Mutant Hunter Assassin is a free solo wargame.

Dunger Racing Game

Dunger Derby is a miniatures game of racing little wasteland beasties called Dungers. You really have to take a look to get it.

Warlord 2085 Science Fiction Rules

Skank Games offers Warlord 2085, a set of free wargames rules for fast play science fiction miniatures.

Giant Stompy Robots Rules

Meca Tac – The Giant Stompy Robots Edition is a set of free wargames rules for Mecha battles.

Planet 28 Narrative Science Fiction Rules

From Mammoth Games comes Planet 28, a set of free wargames rules (actually, pay what you want) for narrative science fiction games.

Mammoth writes:

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.

Link