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.
Rules: Steampunk
Mars Or Die Rules
Mars Or Die is a set of free wargames rules for Victorian era games set on Mars.
Wolsung Steampunk Skirmish Game Rules
Wolsung is a set of free wargames rules for steampunk skirmishes. The company also sells a variety of figures to go with the rules.
Scarf Heroine — Cartoony Steampunk Skirmish Game
Scarf Heroine is a free print-and-play skirmish game with a cartoony steampunkish vibe. There are rules, order cards and cute cutout figures, all free for the downloading and printing.
Wolsung Steampunk Skirmish Rules
Wolsung is a nice looking set of free wargames rules for skirmishes in a steapunk setting.
Update: The previously free rules now are published for sale.