These are some really comprehensive lists here, so I wont go over what has already been mentioned, but here's my 2 cents:
1. Most of you mentioned first aid kit. Make sure you can get to it fast. seconds matter, so for me this is the most important piece of equipment in my Truck. Here in Australia, I'm a Paramedic (EMT I think some of you American blokes call us), & in my line of work, the time it takes you to get to your equipment & also what you have is paramount. So a couple of things on the First aid Kit.
a. Make sure all your stock is accessible & not stacked on top of each other. When you need anything more than a Band Aid, you need it fast.
b. Make sure all your stock is 'IN DATE', out of date stock can fail. More often than not it doesnt, but some do.
c. GLOVES! Usually your only barrier of infection control. Use them. Double Glove if you need to.
d. If you have the money, buy a Defibrillator. They're really REALLY easy to use, but worth ~$5000 AUD. For me, a person's life is worth that much. I carry one in my Truck.
One last thing. Hi Visibility Vest I dont think was mentioned, These are Dirt Cheap. In Australia these are Tax free, so they're worth peanuts from any Safety Shop. Safety First. If you get injured, then you're not much use to anyone.
Hope this helps.
--DuckMan.