The Ralph Files - Voices: Alphabetical listing of character names with the actors who voice them.
The Simpsons Archive: Detailed guide with character lists, voice credits, and celebrity appearances.
The Simpsons Fans at About.com: List of links to cast information, articles and interviews.
The Voice Actor Page: Over 200 credits as contributed by viewers.
Voice Chasers: Cast list with links to the actors' pages.