ISI Citation Indexes (Web of Science)
The ISI Citation Indexes include the Science Citation Index, the Social Science Citation Index, and the Arts & Humanities Citation Index.
ISI allows you to sort articles based on the number of times they've been cited, and to easily move from an article to the articles that have cited it, making it a great search engine for locating the most influential articles and authors on your topic.

Holdings: Science Citation Index Expanded (SCI-EXPANDED)--1945-present
Social Sciences Citation Index (SSCI)--1956-present
Arts & Humanities Citation Index (A&HCI)--1975-present
Updated weekly.