Probably the strongest unit in the game and the reason why so many people think the Terran is overpowered. For example, Terran can build this unit from basically the start of the game. The Protoss counter to the Marauder is Colossi, which are really high up the tech tree (need to get a Gateway, Cybernetics Core, Robotics Facility and also a Robotics Bay before you can build a Colossus; Marauder only requires a Tech-lab add-on to your Barracks). If you're playing Zerg, you basically don't have any good counter for the Marauders.. maybe using fungal growth with Infestors, but that requires like a Hive tech or something pretty high as well. I guess mutalisks would also work, but they die real quick to marines which any half-decent Terran player will always have in his army. Hydras are alright against Marauders but not exactly low-tech. Brood Lords would do well, but they are, once again, at the top of the tech tree.

Reavers - thank god they're not in SC2