If you are going to push your ATV to the limit you are one day going to need tools. Having a good tool-kit can really make life a lot better when repairs are needed. Here is a couple of items I have found to be a must in my tool-kit

Air Compressor
Tire repair kit
Fuses
Sand paper (great for repairing spark plugs)
Zip Ties
Wire
Electrical tape
Flash light
Wrenches
Sockets
Spark plug sockets (5/8, 17, 18)
Knife
Screw drivers
Ratchet
Extension
Screws (Great for small holes in your tires)
Rags