In due course, I plan to write a little piece about the tools one can use to find papers that have been written that cite a given paper. This is one of the most useful tricks a Ph.D. student can add to his or her toolkit. Many professors are closet Luddites who think that their work and the work their friends have told them about "just about covers it."
Well, that's just about baloney. The world is very diffuse, and you can never tell where something has gone once it has been placed out in the wild and wooly scientific literature.
Play with this a little. At first it is very amusing, then it starts to show its cracks. It has poor coverage of probability and statistics, even when there is a tight connection to computer science. The world needs a more comprehensive and unified tool.
For now I'll just drop two big names: The Science Citation Index and MathSciNet. To use them from your Penn office you just go through the library website. This also works from home, though you'll need to deal with a little proxy mess.
More? If you are looking at something that has contact with the larger world, then Lexis and Nexis starts to look big. I won't get into that world --- the librarians are good at that. What I'll add here are the tricks they may not know that are relevant to mathematics, computer science, probability, and statistics.
Also, Zentralblatt is now on-line.