ISI Web of Knowledge (also referred to as Web of Science) is an extremely useful tool for researching. It's a citation database, which means you can use it to find out how many times an article or author has been cited and where.
One of the neat new things you can do with Web of Knowledge is create citation maps (currently in beta, but worth checking out). ISI has recently announced enhanced citation mapping capabilities that let you create a colorful visual representation of citing/cited relationships. Here's an example:
More ISI updates.
