APA Online: the web site for the American Philosophical Association, the main professional organization for philosophers.
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.
PhilPapers: a comprehensive open access index and bibliography of philosophy maintained by the community of philosophers.
PhilSci-Archive: an electronic archive for preprints in the philosophy of science.
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.