I imagine that such extensions would create an image on the fly for you to scan with your phone?

If so, the big problem I see with that is that you have to pull out your phone right there and then. I personally use "Site to Phone" (http://sitetophone.com/) and the good thing is that I can send something from the browser and open it up later when I'm out and about. (it also keeps a history of the saved items)

Compared to trnsfr, Site to phone also has the benefit or allowing you to send the link to a page but also just any text.

I'm not a fan of QR codes. They're also not supported out-of-the-box by any phone with a browser.

I agree with you and I don't feel that QR codes are a good fit for things like this (it's just awkward), but just a small nit: Windows Phones actually do support QR codes (and other bar codes) out of the box. I'm sure there are others, but that is the only know I know of. :)