Science Citation Index: This extremely useful research tool that allows you to find scientific articles which have cited a particular article of interest. For example, the PDB file for a protein provides the reference for the article in which a structure was first reported in the literature. You can use that citation in a Science Citation Index search to find other articles that refer to this article.
CAS-Online is the online version of Chemical Abstracts. If you would like to do a CAS-Online search of the literature, see me.