Mars Or Die is a set of free wargames rules for Victorian era games set on Mars.
Here’s a nice photo essay on building a red planet terrain board.
Edgar Rice Burroughs’ Barsoom novels are among my favorite guilty pleasures. Sean K. Reynolds has a page with material, including monster and character profiles from his d20 Barsoon campaign.
Legends of Old Mars is a Edgar Rice Burroughs’ Barsoom variant of Games Workshop’s Lord of the Rings / Legends of the Old West game. I think that system is perfect for playing games involving John Carter, Tars Tarkas and the other denizens of Burrough’s Barsoom.
Imperial Mars is a set of free miniatures rules for games set in a Mars that has been colonized by the imperial powers. The author writes:
The level we have set for these actions is fairly small scale, the idea being that the transportation of large armies to the red planet is far too expensive to consider. A company of earthling infantry works out to about 24 figures, which gives a good level of playability. The Martians we use, and thus those used in the body of the rules, are either humanoid or reptilian, this is due to the selection of figures available. For simplicity I will refer to them as regular and large Martians (with or without fries!).
Whilst we cannot claim that these rules are “generic”, how could they be? We like to think that the system is so simple and open ended that any “new” races or weapons can be quickly slotted in. This seems to be so simple that most of our games involve some rule modification for the scenario. Anyway I hope that you enjoy them and at least find something in them of use.
These rules were developed initially for 10mm figures though I can see no reason why they cannot be used for other scales.