APA Online: the web site for the American Philosophical Association, the main professional organization for philosophers.
Digital Text Projects: provides full text of many classic writings in philosophy; from the Institute for Learning Technologies, Columbia University.
Stanford Encyclopedia of Philosophy. The Stanford Encyclopedia of Philosophy organizes scholars from around the world in philosophy and related disciplines to create and maintain an up-to-date reference world.
VoS (Voice of the Shuttle) - Philosophy: another large gateway site for the humanities with a significant section for philosophy; as with many such sites, many links no longer work, but others are kept current; created and maintained by Alan Liu and a team of developers at the University of California, Santa Barbara; provides a way to suggest new links and feedback.
The Internet provides access to a wide variety of information - some good and some less reliable. Use care in deciding which sites to use for research. These are some tips for assessing the quality of a web site. Ultimately, you need to look for currency, relevance, authority, accuracy, and purpose.