The little things can make all the difference!
A good heart-rate monitor/stopwatch combo is one of the best running accessories you can buy and will help you get the most of your runs. You can measure your heart-rate, the length and time of your run and the incline you're running on with a good one. They're also a useful investment for other sports such as cycling and even walking. If you can't invest in one just yet, go for a pedometer or stopwatch so that you can measure how far you run and how long you're out for.
A good pair of sunnies are vital, good weather permitting! Make sure yours give 100% protection against the sun's rays. While you're at it, get hold of some non-greasy, water-resistant (read sweat-resistant) suncream to smother yourself in!
A few tunes can really help relieve monotony, relax or motivate you on a run, and iPods, MP3 players, especially slim-fit light ones, were made for long runs. Make sure you don't get so caught up in Eye of The Tiger or Chariots of Fire that you forget about traffic hazards and lamposts, though!
A light backpack is always handy (you can get ones specially designed for runners with belt and clips fastenings to keep the bag from moving around) for keeping essentials such as keys, water, cereal bars and the like with you.