r_rolo1
King of myself
A few ideas....
The stirrup - made cavalry more effective and horse archers possible.
The keel - made ocean-going ships possible. I note that the Maoris, Micronesians and Vikings all had ocean-going ships well before Civ4's technology trees would allow them.
General staff - introduced around the time of Napoleon, made it possible to effectively coordinate multiple armies. In game terms, you might increase all ground movement by +1.
Telegraph/Telephone - made it possible to coordinate business and government across geographic distances more effectively. In game terms, it would reduce penalties for distant cities.
Fire, maybe?

And about the stirrup: There is a Scythian horseman statue from the VII century BC with it, but it looks that it was forgotten and reinvented by some Central Asian tribe in the III century AD.... And you don't need to know it to do a effective horseback riding: most of the ancient cavalries simply tied the horsemen feet one to another ( Numidians did it, as well as later roman ones ). Stirrup bring another and completely diferent thing: it stabilizes the horsemen enough for him to use the total kinetic thurst of the horseman + horse (+ armor ) in a aimed charge.... Mongols ( with their "parthian arrow" technics )didn't need that, but Sassandid, Bizantine ( with their version of the Parthian/Sassandid cataphrachs ) and West European did.....