I went looking for Steampunk RPG options, and it took a while to collect the information. So, here are some Steampunk tabletop RPGs and info about them that I've collected.
If you're looking for a Steampunk tabletop RPG, at least consider these:
Forgotten Futures is an RPG tailored for Steampunk. There's a lot of extension modules for various kinds of worlds, a Java program for character generation, etc. Lots of fun pictures.
GURPS Steampunk. Not too expensive ($7.99 PDF download). It's GURPS-based. This review of GURPS Steampunk is very positive. Take a peek at their free sampler, if you think you might like it. There is at least one extension, GURPS Steam Tech, "A Compendium of Marvelous Devices for the Age of Steam".
"11 Steampunk Adventure Seeds" by Earl S. Wynn is a set of ideas to "kickstart your next game or RPG match". It's just a short blog post, but you mind find some useful ideas there for starting a campaign.
Steam and Steel is a d20-based book available as a downloadable PDF for $1. This review loved it. However, this is focused on a fantasy RPG (dwarves, elves, magic, etc.) with steam added to it. It does have some general info on steam-based technology that may be useful in other settings.
Victoriana looks very promising. It uses an unusual dice mechanic. The first edition is available as a PDF at little cost. There's a second edition with more material; it's more money, and it has a completely different set of mechanics. Victoriana 2nd Edition Core Rulebook (Cubicle 7 Entertainment Ltd.) was available from Drivethroughstuff for $24.99.
Etherscope is available for not too much money. This review liked Etherscape.
Lady Blackbird is a free plot, but based on travel between planets. "Lady Blackbird is a quick-playing steampunk tabletop RPG that's free to download and Creative Commons-licensed for your sharing and remixing pleasure: "Lady Blackbird is on the run from an arranged marriage to Count Carlowe. She hired a smuggler skyship, The Owl, to take her from her palace on the Imperial world of Ilysium to the far reaches of the Remnants, so she could be with her once secret lover: the pirate king Uriah Flint. HOWEVER, just before reaching the halfway point of Haven, The Owl was pursued and captured by the Imperial cruiser Hand of Sorrow, under charges of flying a false flag."
Castle Falkenstein is a playable GURPS setting but has less positive reviews; one review noted its "Steampunkish magical Earth" but that it uses the older GURPS 3e; GURPS is now 4e and Falkenstein's magic system is completely incompatible.
Iron Kingdoms is more a swords and sorcery and steam thing, which wasn't what I was looking for.
Space: 1889 is intriguing; the European counties now can travel to the planets on the lumineferous ether. I haven't gotten it, but it looks like fun.
OGL Steampunk - another d20 rules set. This one is expensive (I could only find used paper, no PDFs), so I haven't looked into it further.
Victorian Adventure Enthusiast has a Guide (also available in PDF) that lists a massive number of Steampunk-related RPGs, with helpful comments about them. I wish I'd known about it before I started writing this!
Ether Emporium has a long and fairly complete list of options, but not much detail about any one of them.
Here's a review of several d20 Steampunk games, including "Steam & Steel – A Guide to Fantasy Steamworks", Etherscope (with an odd "Etherspace" element), Iron Kingdoms ("more wild wild west than neo-victorian"), Mechamancy ("Further from the true steampunk feel"), Sorcery & Steam ("one of the weaker volumes").
As of January 2011, some items were in development and not ready. They might be ready, or dead projects, by the time you read this.
Machina Obscura "is a project to create a Steampunk Roleplaying Game with Pen & Paper, cardgame and Tabletop characteristics". As of January 2011 it looked like a work in progress.
Clockwork Adventures is based on 4th edition D&D. You need those books; third edition will NOT work. As of February 2011 it has not been released.