Not strictly invented during Alexander's rule (it was his fathers innovation) but the sarissa was one of the biggest technological factors leading to the Macedonian army's success.

The Macedonian army was mostly a highland rabble, largely made up of lower classes at the time of King Philip (Alexander's father). Thus they couldn't afford to deck themselves out in the heavy armour that was typical of the Greek hoplite's, meaning that the army wouldn't have had much of a hope in a battle with any of the Greek city states. Philip's technological answer to this was the sarissa: a spear 18 feet long that meant that the heavily armoured hoplite's could barely get close to the Macedonian phalanx, largely negating the benefit their heavy armour would have in toe to toe combat. Additionally, the length meant that around four layers of sarissae could be brought to bear on the enemy (as in men as far as the fourth row back would still be able to reach the enemy line with their sarissae) meaning the enemy would face an incredibly dense mass of sharp spearheads.

Note that equally important was Philip's extreme level of drilling and discipline he made the army go through. It's really really difficult to get an army moving, turning and performing complex maneouvers when each man has an 18 foot spear. Philip's extensive training turned the Macedonian army from a highland rabble to the best fighting force pound for pound in the world. So intimidating was this sarissa+discipline combo that Alexander got a group of Thracian barbarians to flee from a heavily defended castle simply by doing maneouvers outside their gates.