The one tool I couldn't do my job without is our primary retrieval system. It tracks files, clients, dates, forms, procedures, file locations, supplies, everything. Its intuitive adaptive search protocol is consistently faster and more accurate than conventional searches.
That'd be my brain.
But seriously, the office tool I most miss when it's not available is the Internet. Even putting aside the personal use and the office email, I'm online regularly. In the last few weeks, I've searched the Borough Code, checked fees with the Morris County Judiciary, ordered office supplies, looked up property tax records cards, printed postage, looked at the weather forecast to determine when my boss would need to leave the office, found a physical address for sending documents to a financial institution overnight, and tracked a FedEx package to make sure it was signed for on time. That's just what I can think of quickly. When that connection goes down, the entire flow of the business has to change.