What this is
I’m going to list some resources for doing historical research in philosophy. Many of these resources require a University of Chicago internet connection. If you want to access them at home, you will have to use the University’s proxy server.
Philosophy in general
The Regenstein Philosophy Research page lists a wide array of resources. Of special interest is the Philosopher’s Index (the index of work in our discipline).
The Routledge Encyclopedia of Philosophy is reasonably good for getting an overview of an issue, though I have to confess that I rarely understand encyclopedia articles: they’re too condensed. But each entry has a good bibliography.
The full text of many books published by Oxford University Press is available in full-text editions on the Oxford Scholarship Online website.
InteLex PastMasters is a text database of philosophical writing. That means you can search for a word or phrase in Hobbes's published writing and even his correspondence. Oooh. This is a very good thing.
In addition, there are two collections of facsimilies of original editions: you can search by titles and, in some cases, content and then print out a copy of the original pages. Eighteenth Century Collections Online and Early English Books Online, which concentrates on the 17th century and earlier.