It's not free, but if you want some unique and thematically fun game mechanics, look into Deadlands.
If you were a fan of 2nd Ed. D&D, you'd like the original better (crunchier, grittier, more time-consuming game mechanics, more-fun mad science, but kind of a meat grinder). If you don't mind 4th Ed. D&D, you'd like the new Savage Worlds version better (streamlined, way more balanced and fair, but introduces the 'important people' vs 'mook' concept that some people hate).
Deadlands is what happens if you introduce the supernatural into the Civil War and run with it for a couple decades. At one of the major battles, suddenly an army of the dead showed up; all kinds of horrible shenanigans crop up at sites of major violence, so the war never quite ends because there's no decisive battles. The discovery of "Ghost Rock" fuels a steampunk revolution, complete with Mad Science; "hucksters" beat the devil at cards to cast spells, and of course there is some amount of divine and Indian magic too. The Texas Rangers and the Pinkerton Detective Agency are the South and North's respective supernatural-hunting agencies, and the Rangers' motto is "Shoot It or Recruit It". The main tabloid paper, of course, covers real supernatural incidents (sometimes).
Die-rolling uses the exploding-dice thing that's so much fun (rolling 3d8 for a skill? Take the highest number, that's your result. If you got an 8, reroll and add: 4, 7, 8 -> reroll the 8 and get a 5 -> your result is 13). Initiative is based on playing cards: Each round, you draw x cards based on your quickness roll, and you can act in initiative order counting down from ace...but you don't know what the bad guys are holding 'til it comes up. It runs faster than you think, too. Hucksters cast spells with actual poker hands (draw X cards, build the best hand you can, pray you don't get a black joker). Bullets HURT...there's a lot of "the quick and the dead", but you're better off taking the third option of "the smart". Still, duels at noon are a ton of fun and have some good mechanics.
Also, in case you got any ideas about the supernatural elements of the setting being a battle of Evil against Good? No, you're wrong. It's Evil against...uh...well I guess you're hosed, because it's pretty much all evil. The bad guys feed off of fear, you see, and unless some plain old human heroes step up, that fear's just going to keep on multiplying.