Chlorine - used for sterilising and disinfecting water - like urine in the local swimming pool and causing green hair - no probs
Iodine - a disinfectant - also used for making Nitrogen tri-iodide, great for school doorknobs
Fluorine - our teeth were really crappy before it was added to water.
Bromine - not sure about this but heard it was added to soldiers food to de-randify them.
There are of course the naysayers but in suitably quantities these improve life in general and save many lives. Anything in excess causes all sorts of problems - politics, bread, grog, sunshine and the list goes on.