In my philosophy classes I try to use on-line readings when possible. It saves students some money and it provides one with a searchable text. There are many sites where these philosophical texts can be found, but they vary in quality (esp. when the texts are translated).
I'm listing here some of the sites with classic texts in philosophy that I like to consult. I won't link to Episteme since, though it has been an excellent resource, that site became corrupted and I haven't heard that it has recovered.
A. Early Modern
First, it appears that the more important philosophical texts in the early modern era are the most widely available on-line. Here are some of the relevant sites:
1. This Marxist site has lots of texts by Hume, Locke, Leibniz, Spinoza, Berkeley, Galileo, Newton, Descartes, Adam Smith, etc. (They also have works by Heidegger, Carnap, Godel, etc.).
2. This Idaho site has many early modern works, including lots of Locke, Kemp Smith's translation of Kant's Critique of Pure Reason, Latta's translation of Leibniz's Monadology, and works by several of the lesser figures of that period. In fact, the list of texts runs into the 1800's (ending with Schelling). Translations tend to be older, but respectable. I noticed, though, that some of the links are broken.
3. Jonathan Bennett's early modern texts are very helpful, especially for students. Bennett has posted several texts on-line "with a view to making them easier to read while leaving intact the main arguments, doctrines, and lines of thought." Works by the six big early moderns (Descartes, Spinoza, Leibniz, Locke, Berkeley and Hume) are included, as are works by Adam Smith, Richard Price, Newton, Thomas Reid, Hobbes, Malebranche, Anne Conway, Jonathan Edwards, etc. There are even excerpts from Kant's first Critique and a few texts by John Stuart Mill.
4. Berkeley's Essay Towards a New Theory of Vision.
5. Finally, here are the complete works of David Hume.
B. Kant, Hegel, Kierkegaard & Nietzsche
As mentioned, some of Immanuel Kant's works are available at the above sites. One can also find a recent translation of the Critique of Pure Reason by George Macdonald Ross. Here is the orginal German version, and here's another copy of Kemp Smith's 1929 translation; this last version includes a "PERL driven search engine." Stephen Palmquist has put together a glossary for this work.
For Hegel, here is a recent translation of the Phenomenology of Spirit by Terry Pinkard.
Here are some of Kierkegaard's books and several works by Nietzsche.
Walter Kaufmann's lecture, 'Sartre and the Crisis in Morality'.
C. Analytic Philosophy
Much of Bertrand Russell's work is on-line.
Selections from Dewey, James and several philosopher-psychologists (inc. Wundt & Stumpf)
'Scholarly editions of Wittgenstein's works and Nachlass' from the University of Begen's archives, along with many essays about Wittgenstein. Other 0n-line works by Wittgenstein appear in the right menu of the blog 'Methods of Projection'.
Lots of Wilfrid Sellars' works are available, including Empiricism and the Philosophy of Mind, among other Sellarsian texts and resources. There's also a pdf of Sellars' Notre Dame lectures (of which there's an incomplete audio recording).
The HIST-Analytic site has many papers and books (or parts of books). Many, but not all, are pdf's. There are several selections from works by Mach, Russell, G. F. Stout, Pritchard, Moore, Broad, C. I. Lewis, Schlick, Carnap, Reichenbach, Hempel, Ramsey, H. H. Price, Dray, D. C. Williams, Hart, Grice, Rawls, etc. Elsewhere, there's Herbert Spencer's First Principles.
Volumes 1-14 of Minnesota Studies in Philosophy of Science are now on-line.
I found the penultimate draft of John Searle's 'Minds, Brains, and Programs'. Many recent on-line papers in the philosophy of mind are linked to at David Chalmers' site.
If I teach formal logic again, I'll consult this Handbook of Modal Logic as well as Ryckman's Logic Works.
D. Ancient & Medieval Philosophy and Philosophy of Religion
I don't teach ancient or medieval philosophy, but here are two good lists of resources (the latter of which includes many links to modern texts on religion as well). Here's a site that has 'Christian classics', including many medieval texts. This Fordham site gathers links to medieval humanities texts.
I'm sure there are lots of other on-line translations of ancient Greek and Roman philosophers' works, but this is a good start. I found only some old translations of Plato (inc. Jowett's translations) via these two sites. And here are some of Jowett's Plato translations for the iphone. I also found W. D. Ross's translation of Aristotle's Nicomachean Ethics. John Holbo has posted his new translation of Plato's Euthyphro, Meno and Book 1 of The Republic.
This site lists more than 200 modern philosophy of religion texts that are on-line (including many books), including some by Tillich and some by William James. Speaking of James, here's his Varieties of Religious Experience. The Mead Project has posted excerpts from some of A. N. Whitehead's books. Whitehead's Religion in the Making is available elsewhere.
Here's a pdf of Ian Ramsey's paper, 'Talking of God: Models, Ancient and Modern'.
At the Alex site you can search by author's name for e-texts in the humanities, including many philosophical works. I'm not sure how widely the searches range -- many of the above sites don't turn up there -- but it does at least locate works that are available on Gutenberg. Also, the University of Adelaide's library has a list of e-texts in the humanities (again with many of the great works in western philosophy) that are accessible via that library and via other sites, too (inc. Gutenberg). Finally, many on-line classic philosophy works can be located at the Online Books Page.