Affiliate Me With Google Toolbar's AutoLink
In the Beta, the ISBN links are to Amazon, although it's likely that users will be able to tune this in the final release. I can see why many webmasters will feel uncomfortable with this ability for Google to effectively rewrite pages, and this forms one of the major arguments in favour of allowing webmasters to opt-out. But then, Firefox extension coders revel in developing such tools which can, in many cases, provide a useful service.
At the risk of upsetting everybody, Google could always allow either webmasters to embed their preferred book reseller and affiliate code in page headers, so that IF the Google Addlink is used, the original site at least has some influence where the link goes, unless ovwerridden buy the user, who should also have the same freedom. This suggests a default hierarchy where user prefernces over-ride site preferneces, which in turn over-ride Google's defaults.
It would also be nice if as well as pointing to booksellers, the toolbar could be configured to take users to their local library... I'm thinking of a service like Jon Udell's LibraryLookup tool, of course...