I'll second the bungee cords -- could not live without them for so many things.
Heavy duty zip ties, bought at an outdoor store. Fantastic for quick fence fixes.
Black electrical tape for tack quick fixes -- makes a great end "keeper."
Bottled water for everyone on drives and at shows. Easier to keep close at hand than a water bucket.
Multi-purpose tool -- has knife blades, rasps, bottle-opener. These are very inexpensive at any hardware store and make great gifts. We keep one in every vehicle, one in our spares kit, one in the grooming supply tub, etc.
Sharpie pen -- great for hoof touch-uups, bits of bleached mane, etc.
Just-a-Groom trimmer or cheapo beard trimmers for trimming ears, muzzles, etc. Cheap and disposable, but the Just-A-Groom is equivalent to a 40 blade at its closest setting.