Theoi Greek Mythology is an internet encyclopedia with over 1500 pages on various characters from classical myth, covering everything from famous gods and goddesses to obscure nymphs, titans and monsters. If the confusing familial relations of the Greek gods vex you, there are 10 different family trees to help you make sense of it all. There's also an extensive library of ancient works concerning classical mythology and a bibliography should you long for more to read. Last but not least, Theoi has a gallery of over 1200 artworks from antiquity, which I have been happily browsing for a good while.
Sing to us, O Muse, of our Timeless Myths. A site dedicated to Classical, Norse & Celtic mythology and Arthurian legends.