Get wet!  It’s actually pretty fun to run in the rain.  Some pieces of gear are necessary (or at least useful) during a downpour.  Wearing a hat will keep rain out of your eyes.  Using a raincover for your pack will keep everything dry.  Some wet weather clothing helps, such as a goretex running vest or jacket, but in certain conditions they can make you overheat, so pay attention to your body and make adjustments as needed.  Drymax socks will wick sweat and rainwater away from your feet.