Early Modern Philosophy
Online versions of works by Berkeley, Descartes, Hobbes, Hume, Kant, Leibniz, Locke, Reid, and Spinoza. Somewhat limited, historically, but still useful for assigning readings to students.

Internet Sourcebook Project
This is an excellent site for elecronic historical texts from the ancient through the modern period. Based at Fordham University, New York.

Institute for Learning Technologies: Digital Texts Projects

Online versions of some of the great works in philosophy. Philosophers include Aristotle, Descartes, Hegel, Kant, Locke, Mill, Plato, and Rousseau. Also includes links to other digital text projects online. Great for assigning different reading to students.

Philosophers’ Imprint
Features philosophical essays. Most are fairly technical. Could be useful for higher level students or teachers for background or notes.

Stanford Encyclopedia of Philosophy
Alphabetical entries on philosophers, terms, etc. Very useful for students and teachers. Articles can be fairly lengthy — great for research.

And now for something a bit fun:  Beyond the Fringe on Oxford Philosophy, thanks to the wonders of Youtube.