Technically, Spartan Games makes Firestorm Armada, a space combat game, and Dystopian Wars, a Steampunk game based on the same rules engine. They also do a fantasy naval combat game called Uncharted Seas.
For Fantasy, you might also try Warmaster. Strictly speaking it's 10mm, but it is an army scale combat game based on the WFB universe. The rules are free on the GW website, in the Specialist Games section. (You'll also find the rules for the aforementioned Battlefleet Gothic, or BFG, there.) Kallistra makes the Hordes and Heroes range of 10mm fantasy minis of you can't stomach the GW prices. The Warmaster ruleset is good enough that there are about half a dozen or so games from at least three companies that use the "Warmaster engine", the core mechanics from the game. Future War Commander/Blitzkrieg Commander/Cold War Commander are the most relevent to the discussion, being designed for 6mm to 15mm army scale combat.
Of course, you could just play Epic...
Oh yeah, and 6mm is cheaper to start than 28-35mm, but if you're anything like me, it just means you're armies are bigger!