For some things you have wikipedia, but for other a well-maintained directory is the only thing on the top.
Theoi.com is a site that not only catalogs the ancient Greek myths, but also refers to the passages where everything is mentioned, gives you a latin pronunciation, a Greek transliteration of the name and a ton more info. It is an amazing starting point to do your own research.
The Theoi Project, a site exploring Greek mythology and the gods in classical literature and art. The aim of the project is to provide a comprehensive, free reference guide to the gods (theoi), spirits (daimones), fabulous creatures (theres) and heroes of ancient Greek mythology and religion.
Amazing work, support them on their site.