Iron Kingdoms is great, but only nominally steampunk. Its really more "Full Metal Fantasy" to use their terms. The tech is fully magical, and aparently has not really advanced for hundreds of years. Though I love the setting it does requiore some suspension of disbelief, but being a fantasy game that should be easy enough.
Forbidden Kingdoms from Otherworld Creations is pulp, and does have some alternate science material in it. Some disagree with some of the alternate rules systems, and the new classes presented are not necesarily to formula, but the game is very interestig, and one of the most readable game books I own.