"Cited by" feature (i.e., Articles Citing This Article) for PubMed can be downloaded onto the FireFox browser.

"This script takes each result in a PubMed search and then queries Google Scholar to get the number of times it has been cited. Then it places a link with this info next to each search result, linking to the citations in Google Scholar."

http://www.plutosforge.com/blog/projects/ncbi-scholar