Here is a list of websites that I find useful for questions related to English:
Forum to ask or answer questions regarding advanced aspects of the English language. Highly recommended, but do not dare to ask people to proof-read your homework.
A forum better suited for beginners or intermediate users of the English language.
Free, English-only dictionary that provides pronunciation in both phonetic spelling and audio format.

I also recommend the following books:

Practical English Usage, M. Swan, Oxford University Press
A collection of popular issues regarding word use and grammar.

Longman Pronunciation Dictionary, J. C. Wells, Pearson Education Limited
The standard reference for English pronunciation, includes statistics about regional variations.