Web of Science
(Searches through citations, subject indexing, and some full text)
A massive index of literature from the sciences, social sciences, and humanities. Also offers cited reference searching to see who has cited a particular document. 1898 - current.
Note: To do cited reference searching, click on "cited reference search," fill in an author's last name and publication date, figure out which of the resulting list of citations match the work you're looking for, and then "finish search" to see who has cited that work.