The Perseus Project - This is probably my number one on-line source for Greek mythology. The
Perseus Project holds a treasure trove of texts from Greek Myth. Some of the texts are even in
Greek for those seriously studying the mythology.
The Encyclopedia Mythica - This is another great site hosting not only a great amount of information
on Greek mythology, but also a multitude of mythological information from other countries (Roman,
Egyptian, Nordic, etc.).
Bulfinch's Mythology - This site has a good amount of information on the more significant stories of
Greek and Roman mythology. The way the information is presented is in the form of stories rather
than straight up resource information.