I keep a 14" pipe wrench on the back seat of my car. The thing is 3 solid steel parts and more than half an inch thick in some places. It has the practical use of loosing stuck nuts, too. I also keep a police issue flashlight, the old Maglite skullcracker kind, in a pouch behind the passenger seat. It has a super bright LED in it too so it's practicality should be obvious.
When I'm walking around or on my motorcycle I'll sometimes have a knife on me but recently I've started carrying a smiley instead. I just don't like knives.