Essentials

Intersting side project from the group. 

Aside from compulsory safety critical type stuff, what's your most essential expedition item?

We've seen quite a few people doing the TGO challenge (a self supported walk from east coast to west coast, where you decide your own route, schedule and style of trip) and a few mountain bikers and a runner so answers varied!
For me, its a proper toothbrush. Brushing your teeth makes everything better. True fact.