The one tool I cant live without is......my brain. Not a joke. There are so many things we do as IT on a daily basis, that we can do with our eyes closed. There are things we can do with any number of tools (ie software etc..) its merely a matter of choice.

To be able to logically think through a situation, logically attack an issue under pressure, that's what often times makes the difference.

How many situations have you been in where you have come across a tough problem, and you have to take a step back and say "alright, lets think this through" and then WHAM here comes the solution?

1 Tool. Brain. The software is what you use to execute. The Brain is where you memorized the process