[This] will produce Code 3 of 9 barcodes (source [here]). These permit reliable encoding of spaces, digits, letters of the alphabet and the following punctuation: '-', '+', '.', '/'. Certain other symbols are also included in the standard, but although the script will encode them correctly some barcode readers may treat them as parts of extended characters, so their use is not recommended and I will not list them here.
When printed, the image as supplied by the script must be arranged such that it is at least 0.25" high and (0.45" + 0.13" x number of letters in the encoded text) wide or it may not scan (e.g. 0.25" x 1.1" for a five-letter code). Note that this includes the blank area around the barcode in the image. The background does not have to be completely white or even uniformly coloured, but may not contain blue or green colour components.
[This] will produce EAN13 (that's the ones you see on the backs of books) barcodes from ISBN?s (source [here]). That's right, Google-searchers, [this] is the place to go if you have an ISBN and want a barcode made out of it.
Same colour restrictions for background as above. No guidelines for image size AFAICT, but you probably want them an inch or so across at least.
It's worth noting that LaTeX? has quite a sophisticated barcode generation system as well. --Admiral
Method: evilly screen-scrape title etc. from book vendor sites that allow search by ISBN. Online version [here]. Yes, that's right, if you came from Google and are looking for a way of looking up ISBNs, it's [here].
Source [here]. Has now been updated to try lots more sources, in rough order of performance and reliability. Depending on the book's obscurity, searches may take as long as a minute or two. If anyone can come up with an ISBN that it cannot find which Google can, MoonShadow would be quite interested, especially so if the book was published in the UK/US and in English.
Can it find book format (paperback/trade/hardback) too? --DR
Unfortunately, not all the sources list that information. Most seem to list the publisher, if that helps. - MoonShadow
Is that source up-to-date? It seems not to be, on the basis that the Amazon regexp doesn't work properly in the downloadable version (it missed that the relevant meta header ends " /> not just ">. --Emperor