Trying to make lists is always hard work. Each person has different experiences and what may be useful for someone can be an absolute useless for another one. What follows is a personal list about what I use for a race, although I would like to read in the comments what is useful for you, before your training or competition.
Bananas. First part of breakfast, it is a useful source of carbohydrates and potassium (to prevent cramps).
Chocolate milk drink. Useful for hydration, and rich in carbohydrates also. About 500 millilitres, combined with 2 bananas makes for easily digestible breakfast 2-3 hours before race time.
Vaseline. Many uses for a runner. In your feet to avoid blisters, and maybe also in armpits, waistline and nipples. In a long race, any rub can become unbearable painful.
Energetic gels. To keep your glycogen levels and avoid a decrease in your energy (or the wall). For a marathon, I usually take 5 gels, one every 30 minutes after the first hour.
Gels carrier. Not every running pant offers a big enough pocket to carry your gels. Lately I changed my mobile holder for a running belt, that accommodates them easily.
Sunglasses. A “must” for me. I can´t remember the last race without them.
Mobile phone. Always connected, probably a race could be the ideal time to forget about your phone, right? Yes, but facilitates to contact your family/friends after the race and offers a help line if running a trail. You can also listen to your music, although I like to enjoy the surroundings.
Running mobile holder. If we run with the mobile phone, we need something to carry it. I go for an armband, that you can find cheap in every size.
Money. It is not a bad idea to carry some money with you. A note is weightless and may be useful. I always carry one inside my mobile phone (if you manage to open its back).
Small backpack. To keep some dry clothes, keys, etc at the finish line (if it is possible, or you are with someone). And a small plastic bag to keep your dirty wet clothes apart.
Wet towels. Not many races offer showers facilities. They can be handy to clean yourself until you arrive home, sometimes even hours later, if you have travelled far for the race.
Protein bar. To eat after the race and help muscular recovery. It can be replaced for the previous chocolate milk drink, as they are also rich in proteins (besides the carbohydrates).