NTFS Drive Protection is a free and portable tool which can lock a USB drive, preventing anyone from editing its contents (that includes creating new files or folders). The main aim is that your drive won't then be able to be infected by autorun viruses, but the program could also be useful whenever you want to create a "read only" USB key, where its contents can be viewed, but not changed.
It only takes a moment to set this up. Launch Ntfs Drive Protection, select your USB drive in the "Target Drive" box, click "Start Protection" - and that's it. In just a few seconds the program will change your file and folder permissions to prevent them being edited.
If this all seems a little too inflexible, check the "Create an Unprotected Folder" box, and the program will then create a new folder (\_Unprotected by default) with the standard file permissions. And so any existing files or folders will be locked, but you'll still be able to save new files in the_Unprotected folder.
If you don't need this drive protection any more than clicking "Stop Protection" will remove it.
And an Options menu provides quick access to various drive-related options: the "Disk Management" applet, the "Safely Remove Hardware" dialog, drive properties, format and more.
One problem with all of this is that it relies on your drive using the NTFS file system, which isn't generally the best choice for USB keys (it's less compatible, and writes more to the drive, increasing wear and shortening its life).
It's also at least possible to change the permissions on a non-removable drive (though not the system drive, fortunately), which could cause you all kinds of odd problems. You're most unlikely to do this by accident, but it's still a program you should keep well away from people who don't know what they're doing.
NTFS Drive Protection does provide a quick and easy way to lock the contents of a USB drive, though. It should be enough to ensure viruses can't infect the drive with their own autorun.inf file, and if you're only transferring files between PCs then there probably won't be any compatibility issues. Give it a try, see how the program works for you.