I like tools. I like having the stuff to do other stuff with.
In the car I have a flash light and an oversize 1st aid kit.
In my backpack I carry a full size leatherperson Core, lock picks and batteries
On my key chain I carry a leatherperson micra (I feel the same way (Leatherperson warranties are the best, I highly recommend their products)), a 4GB USB drive with tonnes of troubleshooting apps, and a red LED light. For a while I had a tiny little maglite on there as well that I just loved. Unfortunately it wasn't very well designed and fell off while riding to work last week.
What kind of tools do you keep around?