Library DIY

In every discipline there are journals that scholars consider highly influential. Journals are judged by various metrics, including some that assess the "impact" of published articles. These metrics usually look at the number of cited references made to papers published in a given journal.

Citation metrics have received some criticism because they are not a clear measure of quality. (For more details see the Declaration on Research Assessment and the European Association of Science Editors Statement on Inappropriate Use of Impact Factors.) 

Journal Citation Reports
In the library database Cabell's, search for a specific journal to see its impact factor, or select Advanced Search (Disciplines & Topics) to view journals in a specific subject category with impact factors. 

More on how impact factor is calculated (article from Journal Citation Reports)

Eigenfactor
Eigenfactor measures the impact and importance of science journals in the scientific community. On their website, you can search for a specific journal, search by discipline (choose an ISI Category), or search by year. 

How Eigenfactor is calculated (article)  

Results displayed in Google Scholar contain show a "Cited by" link beneath the article listing which shows the number of sources Google Scholar found citing the article. More current articles won't be cited as often as older articles:

Google Scholar Metrics allows authors to view journal rankings and ratings by various h-indeces. Journal ranking can be viewed for the top 100 publications in 9 different languages, or by broad subject research areas and numerous subcategories. Scholar Metrics uses those articles published 5 years prior to current year indexed and citation from all articles indexed in Google Scholar. 

The Google Scholar metrics section is accessed from the search page: