Earth Expects is a set of free wargames rules for space combat. The authors write:
Earth Expects is a highly detailed set of tactical space combat rules which emphasise skill over firepower. It is well suited to playing battles from duels to squadron actions; because of the high level of detail large fleet battles can be slow to play and therefore optional rules are provided to speed play in these games.
Titan Rampant is a free variant for the commercial Dragon Rampant rules. Ownership or access to Dragon Rampant is required for play.
Exoshift is a set of free wargames rules for mass battle science fiction games.
Combat Squads is a set of home rules for gaming in the Warhammer 40K Universe. From the rulebook:
Combat Squads is an attempt at creating a set of competitive skirmish rules for Warhammer 40.000; a game set in the grim darkness of the far future.
Within this booklet you’ll find rules for constructing, and playing, with small Combat Squads using the miniatures you already have for your regular Warhammer 40.000 army.
As such, then you’ll need the core rulebook for Warhammer 40.000 (currently, the 5th edition rules) in order to play Combat Squads. Furthermore you’ll need this booklet, of course, and the codex for your chosen army.
The rules presented here in the following are intended to supplement your normal games of Warhammer 40.000 giving you a chance to play the odd one-off special mission during a campaign game. However, they have also been
written in such a fashion that you could play Combat Squads as is; with an inbuilt campaign system as well as a slew of new missions.