For the study of language, Prof. Graves recommends the following educational sites:
The Learner's Dictionary is great for hearing English pronunciation (especially when working on phonetic transcription), identifying parts of speech, finding different forms of a word that you need, and ascertaining usage, such as which prepositions go with which word.
American Tongues: watch clips/American accents and their social implications
The Speech Accent Archive: regional accents in IPA, with recordings
Dictionary of American Regional English: Do you speak American?