I am sure that everyone has some excellent tools in their camera bags, but the following 3 tools have saved us multiple times on set:

1. Leatherman - I have the Free P2, it has all the gadgets you need!

2. Clamps - we use clamps to keep things in position or to hold up our blackout material

3. Gaffer tape - helps with light leaks, securing cables, and holding up paper/cloth, it's pretty strong and leaves little to no residue