A Brief OverviewThe popular and revered G.A.S.L.I.G.H.T. rules support a wide range of adventures from Victorian Weird Science to Rayguns and Rocketships pulp.
Adamant Entertainment offers both The Imperial Age series (d20 based) for Victorian Adventure and MARS (d20, Savage Worlds) for Barsoomian amusements.
Speaking of interplanetary colonialism, the venerable Space:1889 covers Ether-space from Mecury to Mars and all points between.
Blacksburg Tactical Research Center brings us Verne for their EABA system. Verne is well-grounded in both the fiction and the fact of the Victorian Era. As a bonus they provide a free character generator.
From Cubicle 7 comes Victoriana and Abney Park's Airship Pirates. Victoriana blends classic RPG magic and fantasy with Victorian Romanticism while Abney Park's Airship Pirates looks to a dystopian Neo-Victorian future based on the band's music.
Osprey Wargames presents In Her Majesty’s Name: Steampunk Skirmish Wargaming Rules. A fast-paced game emulating the climactic confrontations of literature and cinema. Two or more Adventuring Companies set-to over a mutual objective. Steampunk tech and Mysterious Mystic Arts (usually from the Near or Far East) are all included.
I use a mash-up of Two Hour Wargames rules including Colonial Adventures and Larger Than Life. I've even thrown in All Things Zombie on occasion.
History of Steampunk & Victoriana RPGs
In his blog, Age of Ravens, author +Lowell Francis posted an exhaustive eight-part listing of RPGs with Steampunk themes. He covers the years 1983 thru 2012 in a thoroughly informative and entertaining manner.