A handheld travel scale for weighing luggage ($10). — Recommended by Mark and Mimi Comfort, who run the Cruise Holidays booking agency in Kansas City, Missouri.
I particularly like the cell phone and laptop extended batteries and hand-held water purification system (similar to an iStraw). Some other good ideas include an Ultimate Spork, a laptop lock that can be used to secure many things – including luggage, or an ultimate jacket.