You know its hard times when you end up in the tool aisle of a 24hr grocery store.
Lucky for me there is a 24/7 Walmart 15 minutes away.
i guess we should know if the OP is refering to hand tools, test equipment or specialty tools. any of the three will get you a different answer.
I was thinking hand tools - somehow I regularly find myself in need of the socket that's not in the set (because "nobody uses that size"), or the universal is the one I busted last month, or I can't figure out if my 11-year old son hid it, or I left it in the rotting volvo seat at Pull-a-Part...
I was also obviously thinking of home use. At work, if the tool ain't there the job stops until it gets ordered & delivered (tools are company supplied).
I usually try to make it out of a similar tool.