Multivitamins and Minerals
Because most peoples diets lack the proper nutrition (whether due to poor diet or foods generally lacking nutrients in the quantities we need). Unless there is an underlying medical issue, most people need a vitamin & mineral supplement. Be sure to review the ingredients and consult your doctor before starting a new supplement to make sure there isn't anything that will hurt you in it.
For the regular Joe and Jane Fitness, I usually recommend a vitamin supplement like NOW brand Adam (for men) or Eve (for women). They are easy to take and inexpensive.
For beginner and intermediate athletes I recommend Orange Triad. OT has pretty much everything you need and a joint supplement all in a convenient regiment (3 doses a day).
For intense athletes, I turn them to Universal Nutrition's Animal Pak line. The Pak is 11 pills (yes that's a lot) that have been formulated to time release for optimum efficiency. The Animal Pak is not just a multivitamin though, it also has digestive enzymes, BCAAs, Antioxidants, and Performance Blends. The Animal Pak line also has other "add on" supplements such as joint support (Flex) or Pre-workout (Rage) that are formulated to work best with the Animal Pak.