Über RPG: Steampunk

From the publisher:

Welcome to a world of mad, inspirational wonders and wild scientific theory wrapped in fog, run by gears, and lit by gaslight. Steampunk is a venue for adventure, no doubt about that, with science and technology unhindered by the laws of physics, with societal norms from any culture past, present or future, and with only as much historical accuracy as fits the whims of the moment. It is a realm where anything is possible under the guise of skimming whatever we want from an idealized – or grossly imperfect – version of the 19th Century.

This is the core rulebook with everything you need to get started.