It's funny how mechanical items choose the most inopportune times to fail. We've all experienced what happens when our rig, "Old Faithful," decides to break a belt or suffer some other mechanical malady deep in the backcountry. For this reason, it's a good idea to pack along a new serpentine belt as well as extra engine oil, transmission fluid, and gear oil. Some of us pack along the major stuff like extra axleshafts and hubs, but what happens if something much simpler goes south, like if your battery pukes? Jumper cables won't help if you're alone, so it's a good idea to pack along a self-contained "jumper box" like the Black & Decker Electromate 400. This rechargeable unit has 12-volt jumper cables so you can get going again. As a bonus, it also has a built-in air compressor and a fan-cooled inverter, among other things. Packing basic handtools is also a must, and if your rig takes vehicle-specific tools, make sure you have those along as well.
When you say goodbye to civilization, it's OK to take a little of it with you. Zip-ties, duct tape, and a small roll of baling wire are worth their weight in gold if you have to MacGyver your rig back together. Another mandatory must-have is Rescue Tape. This stuff is like duct tape on steroids. First used by the U.S. military, it is self-fusing and boasts a 700psi tensile strength. It insulates 8,000 volts per layer, withstands 500 degrees F., and is flexible to -85 degrees F. Not only that, it resists most every oil and chemical and creates a permanent air- and water-tight seal in seconds. Of course, your safety is critically important above and beyond everything else. For this reason, you should pack a fire extinguisher and a first aid kit. Bring along a CB radio or some sort of two-way radio for short-range communication. A cell phone is a must for long-range communication. And if you're not a person who uses text messaging, learn. Unlike voice calls, text messages can be sent with just minimal signal. And here's the most important thing: get a GPS unit. This will allow you to pass along your latitude and longitude if you're lost, broken down, or injured. There have been instances where rescuers could talk to folks via radios and cell phones but couldn't get to them in time because they couldn't find them. It doesn't have to be an expensive GPS. We always pack an entry-level Garmin Etrex unit that offers basic operations, including latitude and longitude.
Kidde fire extinguisher | 
Rescue Tape | 
First aid kit |