Springtime is the time for swarming bees! Many times this happens because a colony will have enough members to slit into two, hence creating the need to find a new home. The only reason for swarming is to find a new home, not to scare you and all of your neighbors. Swarming bees are actually quite peaceful. Once they sting you they lose their lives, so obviously dying is not their goal. Just call a local beekeeper--- many police departments have a list of beekeepers that would be happy to come and remove the swarm. Whatever you do, don't spray them with insecticide/wasp spray. Bees are too valuable to the environment to kill them.
Here is a picture of a swarm the Hackenbergs found just recently!