These metrics list the h-index of a publication (rather than a researcher) and are based on the Google Scholar database. One advantage of the Google Scholar data is that it includes web repositories like the Social Science Research Network and arXiv. One disadvantage is that you can't see a title's ranking within a subject (unless all publications in that subject share a common keyword in their title). See the help page for more information.
Some Journals report this information on their websites. Check the 'about' page for each journal.
Other resources to try: