We all have something that we could never leave at home when the open road beckons, what's yours?
I'm never sure whether the mammoth travels with me or I travel with him but neither of us seems to want to travel without each other, we do however always take one piece of kit with us wherever we go..... a corkscrew/bottle opener, well I do live with a mammoth!
I always take a computer. With the high availability of public wifi (at least in developed areas) it's been an incredible resource for last-minute planning, and helped my road trips becomes much more spontaneous. That said, with increasing electronics searches at borders, having one used just for travel may be a very good idea.
Computer, cameras, noise cancelling headphones, a variety of charges and converters.
one simple answer without money you can't travel the world.