USBDumper (article is in French; here’s the software) is a cute little utility that silently copies the contents of an inserted USB drive onto the PC. The idea is that you install this piece of software on your computer, or on a public PC, and then you collect the files—some of them personal and confidential—from anyone who plugs their USB drive into that computer. (This blog post talks about a version that downloads a disk image, allowing someone to recover deleted files as well.)
No big deal to anyone who worries about computer security for a living, but probably a rude shock to salespeople, conference presenters, file sharers, and many others who regularly plug their USB drives into strange PCs.
EDITED TO ADD (10/24): USBDumper 2.2 has been released. The webpage includes a number of other useful utilities.