Web of Science (1900+)
Web of Science is the most complete and comprehensive citation tracking database with coverage from 1900 to the present. It provides thorough coverage of ~5900 major journals. Includes cited author searching. Abstracts, keywords, and additional subject categories are included and searchable from 1991+. (Video Tutorial)
ScienceWatch.com - Summarized Publication and Citation Data from Thomson Scientific for the Analysis of Research Trends & Performance. Visual representations of leading scientific literature track the ground breaking papers in various scientific fields.
INSPEC and Compendex
INSPEC and Compendex on 
For all major engineering, computer science, physics and applied science topics these two databases represent the major access point to the literature. This is the place to start any literature search!
Scopus (1996+)
International coverage of journal articles, selected web sites, and patents in the sciences and social sciences; provides citation tracking for journal articles and conference papers from 1996 to the present.
SciVerse Hub - a search and discovery tool for the working researcher.