The CueCat scanner is a cheap but effective barcode scanner that plugs into your computer's USB port. LibraryThing reads data from unmodified CueCats.
Help! How do I use my CueCat?
CueCats ship with a short instruction sheet (PDF), and you can find more information in the FAQ.
LibraryThing user GreyHead has also written a very helpful guide.
CueCats work with LibraryThing without modification. There are also numerous sites that explain how to "declaw" the CueCat, for use with other scanning applications. For more about this, and about the wacky history of the CueCat, see Wikipedia.
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Email infolibrarything.com with questions.