This information is provided with no warranty. The original list from which I worked was created by searching on "tag:economics tag:blog" in del.icio.us. For me to consider a site an economics blog, it generally had to have at least one URL bookmarked with "economics" as one of the top two tags and "blog" as one of the top four, and I eliminated some sites just because they didn't look like blogs to me. I tried to combine the number of saves for multiple URLs pointing to the same site, but there were undoubtedly some that I missed, and the decision as to what constitutes "pointing to the same site" was a subjective one. (I also ignored bookmarks pointing to particular posts within a blog.)
I'm working on a more comprehensive list (maybe the top 150, or something like that), but I may just give up, because this is turning out to be a lot harder than I expected. Maybe it will get easier if del.icio.us develops better search tools.
UPDATE: Corrected spelling of "Posner" and "Harford".