Some items with the most possible uses may include:

towels - Many of their benefits are mentioned in The Hitchhiker's Guide to the Galaxy
handkerchiefs - Just about any scout book will explain many possible uses of this item.
tape (preferably Duct Tape) - Duct Tape has numerous self-proclaimed uses, many of which might actually be practical.
rope (or twine, or string, or something like that) - These items have countless uses as prooven throughout history.
paper - Perhaps not quite as usefull as the other items, but it's usefulness usually also involves some sort of writing implement.
scissors/knife (or other sharp objects) - Often usefull when used properly, but usually confiscated by airport security.