Diagnostic Tools for Testing 2nd Hand Machines?

tom asks: "Buying second hand computers you always run a risk. I was wondering if the slashdot readers could suggest a toolbox of (preferably small) tools that you could take along with you on one or two floppy's so you could run some diagnostics on machines you would consider buying. I'm thinking of the checkdisks, benchmark programs, soundcard checks, USB checks etc."