If you carry a knife in your vehicle, you might consider checking your local laws to make sure it's legal. Needless to say, no one wants to get arrested.
In my state, a knife with a blade of 5 1/2 inches or more is illegal to carry "on or about your person" but a machete is legal. Doesn't make much sense, does it.
I keep a green handled Frost brand mora knife with a 4 inch blade above the sun visor in my truck. It cost about 10 bucks from Smoky Mountain Knife Works:
http://www.knifesearch.com/images/103-2528.jpgI also carry a pair of EMT shears on a lanyard for cutting seatbelts, as well as a spring loaded punch on a second lanyard. I have carried a claw hammer in the past, in the passenger floorboard, but I took it out. I may replace it, now that I'm thinking about it. I do keep a Gerber (Fiskars) hatchet in the cab within reach. My truck is crammed with stuff, and the main problem I have is finding something when I need it.
Re: seat belt cutters- I bought one, then tried to cut a seatbelt in an abandoned junk car. I couldn't get it to work. The edge of the seatbelt material would roll over and the cutter wouldn't cut the double thickness. That's why I have the EMT shears instead. The cutter was from an EMS/police supply place and I assumed it would work as advertised. Was I doing something wrong? This is the cutter:
http://www.firstrespondersupplies.com/mm45423.jpgOnce I had a seatbelt buckle jam. I didn't want to cut the belt, so after some effort I managed to pull enough slack to wriggle out of seatbelt. I'm older now and, ahem, a little bigger around the middle, and I don't think I could do this again. I finally got the buckle to release and found a nickel (a small coin, for our international friends) inside that jammed the mechanism.
Good luck!