Space 1889 was the earliest Steampunk game I know of. In 1989 the genre Steampunk didn't even exist. My gaming group at that time couldn't get behind the concept of a Victorian Age space game, so we never played more than one or two sessions. I still like Space 1889 best of all the games. Today you can even chose between different flavors: the original game, Savage Worlds and Ubiquity (German only).
Airship Pirates, the RPG inspired by the awesome Steampunk band Abney Park. Go and check out their music. It's fantastic. The game runs on a streamlined version of Heresy engine (as is Victoriana). As much as I like the music, I dislike the postapocalyptic setting. Steampunk is in my opinion a fusion of Victorian historical background and a divergent technological angle. Airship Pirates takes away the Victorian values of the setting in my opinion.
Victoriana also runs on the Heresy Engine, but with levels and magic and stuff. I like the idea of "gutterrunners" put forth in the game. The emphasis is more on magic than on technology (at least in the corebook). Also the period illustrations by Gustave Doré are elegantly and effectively used as chapter openers. It's supposed to be a game about conflicting ideals and ideas, but tries (too) hard to be different. It's got dwarves, elves, orcs and the whole gamut of fantasy races. In the end too much Tolkien in a world that is similar to earth, yet not identical.
In a similar vein I dislike Wolsung. The rules are overly complex, fantasy races are used to stereotype ethnic minorities, and the world is still earth-like, but even more changed than in Victoriana. Who ever decided that pairing Tolkien races with a pseudo-Earth makes a great steampunk game? What I absolutely like about Wolsung is the genre discussion, the encounter locations and extensive beastiary.
Unhallowed Metropolis has the right vibe. Carefully modernized Victorian values in a new dark age. One of the creators called the game "Medicinepunk". In my opinion a fitting moniker. The setting description is very thorough and hits the right notes (I especially dig the intelligent discussion on prostitution and corsets for men - but that's just me). The game system is serviceable but a bit overly complex, especially at character creation.
There is one Austrian Steampunk RPG called Finsterland. It is a great effort, especially since it less about Victorian flavor than Wilhelmian flavor. Especially for German gaming groups somewhat more recognizable and an interesting outlook on the own history before the great war. It is set in a fantasy world and uses a d10 pool system.
Last, but not least there is of course GURPS Steampunk. An invaluable resource, even if you do not play GURPS (which I don't). But it discusses character archetypes and possible technological innovations.
All in all, I like something about most of the Steampunk games I own, but none of them are the perfect fit. FATE Core might be of some help here. So I expect to see some posts about Steampunk in the future.