PERSEUS PROJECT (TUFTS UNIVERSITY): Perseus offers a large body of texts in the original, usually with linked English translations, and utilizing hypertext links for vocabulary. The collection includes Greek, Roman, and Germanic texts (i.e. Old Norse and Old English.) The site also offers several lexicons, including the LSJ and "Middle Liddell".
HOMER LIBRARY (NORTHWESTERN UNIVERSITY): This collection is much more limited (Homer, Hesiod and the Hymns) but the stated intention is to provide: "a multilingual database that uses the search and display capabilities of electronic texts to make the distinctive features of Early Greek epic accessible to readers with and without Greek." It has the advantage of offering interlinear text, and is thus a great resource for those who have not formally studied ancient Greek.
Please expand this list if you know of other peer-reviewed resources for mythology.