For an overview on the topic, see the Introduction under the Journal Rankings tab.
Eigenfactor.org
A free and searchable database, Eigenfactor covers the natural and social sciences and "also lists newsprint, PhD theses, popular magazines and more." The Eigenfactor is now included in Journal Citation Reports. It continues to be listed here for use on its own.
The website includes an interactive mapping function that shows the relationship of branches of science to each other based on the size of the field and the citations generated by the journals of the field. Rather than the "soft" categories used in Journal Citation Reports, where a journal may be located in one or more categories, Eigenfactor uses a hard category where a journal can only fit in one discipline.
Harzing.com
"This site provides information, online papers and resources about Harzing's areas of research: International and Cross-cultural Management. It also presents resources to assist with academic publishing and the assessment of research and journal quality, as well as software to conduct citation analysis."
Source Normalized Impact per Paper (SNIP)
Based on Scopus data, SNIP is a European ranking that attempts to address the problems of varying citation rates among disciplines and the lack of statistics to indicate levels of significant differences.
SCImago Journal & Country Rank (SJR)
A free source that uses data from Elsevier's Scopus database. Includes a "compare" feature that compares journal citation among countries. There is also a "map generator" that shows citation relationships by country. The SJR indicator aims to measure "the current 'average prestige per paper'" of research journals and is one of a new set of journal rangings based on eigenvector centrality (Gonzalez-Pereira 2009) .
