Science fiction set in alternate Victorian timelines (Steampunk) interests many people and there are several works of fiction such as Bruce Sterlings and William Gibsons Difference Engine and several of Michael Morcocks books, notably the Nomad of Time trilogy.
There are also several games based on the idea.
Below are a couple of wild ideas from myself and my friend Ed.
A New Small Arms Rules System for Space 1889.
This gives a more realistic and detailed game than the original rules without making things unduly complex. Using this system you can adapt any weapon of the period to the game.
Some Information on 19th Century Small Arms.
Some of this was taken from Albert Lowes gun files at http://www.space1889.org/
Ive corrected a few minor errors and added a lot of additional weapons and some historical and technical information.
Steampunk 1920 Small arms.
Information on additional hardware likely to be found in Steampunk 1920 games.
Some of my ideas on Conveyances
The Imp Maxim carrier
Two Descriptions of typical Martian garb.
Lord Edwards Ideas to Modernize Warfare.
A World shaped by Tesla
Some thoughts on Aerodynes
Not a game site, but plenty of information on some of the weapons likely to be encountered.
By the Author of the Scrapboard :
Attack, Avoid, Survive: Essential Principles of Self Defence
Available in Handy A5 and US Trade Formats.