Awhile back I listed the contents of a Data Center Go Bag that I helped put together: Data Center Support Go Bag. It was immensely helpful for us as my company branched out from a single data center to more sites.
SAGE (System Administrators Guild) also has a list: SAGE Toolbox
What I have found is that all I really need is:
- a sharp knife
- a pen
- a screwdriver with both phillips and flat heads