Dead Simple Steampunk is a set of one-page RPG rules for settings in a Victorian science fiction setting. There’s also a one-page setting and one page for equipment.
The Transactions of the Royal Martian Geographical Society was a Space: 1889 fanzine that has since gone defunct. A good number of articles from the magazine are now available online at the Heliograph site.
Majestic 12 Games has these demo rules for their Iron Stars miniatures wargame. I’ll let them explain:
Iron Stars is a game of space combat in an age of steam and steel. Set in a fictional universe based loosely on the writings of H. G. Wells and other fantasists, it provides a back story and ship designs from an alternate history in which the Martian Invasion actually happened, the ether is all?pervasive, and Cavorite is a reality. But, as with Starmada and other Majestic Twelve Games products, the focus is on players’ imaginations; a clear and concise ship construction system is provided so you can pit your own space dreadnoughts against the likes of the Royal Navy Ether Squadron or the German Aethermarine.
Space 1889 was/is one of my favorite RPG settings. Here’s a short film/teaser set in the 1889 universe.
For those of you looking for a different setting (indeed, a different world) for your steampunk games, you could take a look at Steamworld Daleth. The author writes:
Steamworld Daleth is a gothic world on an alien planet. Call it an imaginary country, a geofiction or a follie. During the many years I have worked on Daleth (now over 20 years) the world has become what she is now: a steampunk/gothic world where the echo of the Victorian Age still sounds, but where many odd things are yet to be discovered. It’s a mix of fantasy and reality, a dreamscape of my imagination. Daleth can be used as a setting for the role playing game Intrigue & Illusions, but is not developed as such. Daleth is much more than a mere setting for a RPG. It’s a world with its own dynamics.