There are obvious items like clothes, medications, shoes, etc. that drivers will be taking on the road with them. But there is only so much space, and considerations have to be made for things like time of year, and type of job.
Every driver will eventually end up with his or her list of "must haves" for truck drivers, but we've got a lot of suggestions for you below.
Keep it small, keep it legal, and keep it necessary.
The main items that you will want to avoid having in your truck are illegal drugs, alcohol, and weapons, especially firearms of any kind. Drivers are legally prohibited from driving until 4 hours after consuming alcohol, and most trucking companies will specifically outlaw having any alcoholic beverages in the truck.
The web of different gun and weapons laws from state-to-state and jurisdiction-to-jurisdiction, along with most companies and customers specifically prohibiting firearms in their trucks or on their properties, make carrying guns or other deadly or dangerous weapons a terrible idea. Keep in mind that some states also ban stun guns, as well.
Getting caught in any of these cases could end your driving career, possibly forever.