This looks like it could be a useful site:
Basically it has a search form on it where you can choose a lens, choose a camera, choose some other specifications like focal length range or aperture range, and it’ll search its database of photos for ones that match those criteria. So, for instance, I picked the Nikon 16-85mm VR, and the Nikon D40….and received zero matches. :) However, each menu also has an “any” option, so when replacing the D40 with “any,” I got many more results. Then you can look at the specs for each photo match or click on a link to view a full resolution version and “pixel peep.”
The problem is that most of these pics appear to be hosted on Flickr. Many, many Flickr users have their original size photos hidden, so when you click on that link it just says “oops, private page.” All you are allowed to see, in many cases, is the 500 pixel wide “medium” version. That seems like a pretty major flaw. You can’t tell very much about lens quality by looking at a 500 pixel image. Even the quality of the bokeh will look different when the resolution is reduced that much.