These are called flea beetles, see http://en.wikipedia.org/wiki/Flea_beetle for more pictures. These beetles often migrate en mass into home gardens from nearby agricultural fields after the fields are harvested. They can damage garden plants but are not otherwise harmful. The best solution is to work with the farmer to reduce the infestation in their fields (contact your county Extension office for their suggestions as well). Second, gardens can be treated with neem oil or spinosad insecticides when the beetles are migrating, this won't stop all the beetles but it may protect some of the garden plants.