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 Journal Citation Reports (JCR), search for a specific journal to see its impact factor, or view journals in a specific subject category ranked by impact factor. Choose either the Science or Social Sciences Edition radio button to see relevant subject categories. Then you can sort journals in a specific subject ranked by impact factor.
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.