Apollodorus - From Perseus
Summary of Apollodorus' Library with links to Loeb translation of text
Classical Myth: The Ancient Sources
Text and image files about the major Olympian gods
Myths and Legends
Annotated and comprehensive list of links to sites on world mythology topics.  Organized geographically.
Mythman
Huge informative site on classical mythology with excellent sections on Roman & Egyptian mythology.  This site  also includes great images and clipart.
Mythography
Comprehensive site includes short essays on the gods and heroes. Good place to start for a quick reference point.
Encyclopedia Mythica
As the title implies, an online encyclopedia of myth - some articles are quite short, but the scope of this site is global.  Other areas include folklore, bestiary, heroes and images.  Use the QUICK JUMP menu on the right to navigate to GREEK MYTHOLOGY.
Mythweb
Cartoon-style site with information about the major Olympian gods and assorted heroes (Jason, Theseus, Hercules, Odysseus, Bellerophon and Perseus).  Essays about the gods are comprehensive and easy to read; graphics are superb.
Greek Mythology:  From the Iliad to the Fall of the Last Tyrant
Use the IMMORTALS to find an alphabetical list of the gods.  Greek names are used, but there is a helpful Greek/Roman name reference.  Information about the gods includes pronunciation help, well-crafted, accurate short essays and citations from Homeric poetry for each divinity.
Characters of Greek Mythology
Navigation of this site is a little unwieldy, but if you scroll down on the homepage, the site organization becomes clear.  Use the Character Index or the topical links at the bottom of the homepage.  Beautiful artwork included in each essay.  Essays are organized consistently with 3 sections "Stats & Facts," "Birth" and "Lovers" which are followed by sections that are more specific to each god. 
Bulfinch's Mythology
An old standard and classic reference for the stories about gods and heroes.  Often includes references to other poetic works that allude to the characters of Greek mythology.  Stories here are told like a story and make for a much more fun and interesting read than the more encyclopediac references above.
Greek Mythology Link - by Carlos Parada
This is an amazing site with thousands of cross-linked references, incredible art and citations of the literary references relevant to each character and story.  Parada uses the transliterated forms of the Greek names, which are not always spelled in a familiar way.  However, the value of this site is immeasurable, and it will provide a useful, detailed point of reference for any student of the Classics, from beginners to professionals.
The Real Thing -
Using the Ancient Sources to Uncover Myths and their Meaning
Glossaries:  Characters & Words and Phrases
Alphabetical glossaries tailored to Morford & Lenardon's 7th edition; useful resource for any student of mythology
The Theoi Project
A uniquely organized "guide to the Greek gods, spirits and monsters."  Includes ample primary source material, and a bestiary.

Tales from Greek Mythology
A stunningly gorgeous presentation - both in writing & graphics.  Good stuff here.