Timeline 300 is a set of steam-punkish rules for a now-defunct line of miniatures. The rules, though, can be applied to other figures.
The second edition Maifaux Rules are available free for the downloading.
Colonials vs Martians is a set of free wargames rules for steampunk adventure. The author writes:
The great behemoth landships with coal fires stoked and steam boilers at full pressure chub their mighty pistons, sending vibrations of fear shaking the ground and hearts of men. Landships are the tanks of H. G. Wells imagination and the main figures in this game, One per side along with 4 (5-10) man infantry squads, gatling gun, and a cannon crew of two men, or limits of your imagination... as you experiment with more squads and/or landships in larger battles. The enemy of our Victorian Era heroes armed with steam and black powder? Martians! These are Martian's not of -war of the worlds- level tech, but who grasp ‘like’ ability weapons they may have a co2 powered bolt weapon or Martian water pistols (to be cool) but just use “musket” stats. How they got to earth in the days of the Victorian colonial era or we got to Mars... um, would you believe natural space worm portholes? How about Martian Either water, that negates gravity, painted on wooden sailing ships? That wacky Ether!
Wild West Exodus is a skirmish game that mashes the 19th century American West with steampunk and horror themes. You can get the free starter rules here.
Cloudships of Mars is a set of free wargames rules based on the excellent Space: 1889 universe. The Cloudships game features ships propelled by steam and sail gliding high above the plains of Mars thanks to the magic of "Liftwood."