Allen Wrenches or Hex wrenches
Some specialty hardware has an internal hex that you must use a male hex on. These wrenches typically come in sets in a nice holder. Available in both SAE and Metric, I recommend a set of each. Pay the extra money and get the ball end version. These allow you to tip the wrench at an angle to reach better, and are easier to use. Straight end ones will only go in straight, and are harder to use.