Here are a selection of resources related to phonology:
Adrian Underhill’s interactive phonemic chart (and pdf version) is useful for trainees and students alike. He is the author of Sound Foundations, a book on teaching phonology, and also has a series of videos on YouTube on pronunciation, as well as The Pronunciation Blog.
Cambridge English Online (not Cambridge English), offers some fun ways to practise phonemes. Shoot a symbol, Which phoneme, Odd phon’ out and Phonemic flip are examples of activities that trainees and students can use to practise recognising phonemic symbols. Note: this site is completely Flash based.
Oxford Learner’s Dictionary and The Longman Dictionary of Contemporary English are two excellent free online dictionaries. Look up a word in either of these sites and you have the option to hear the pronunciation (in British and North American versions), to view the phonemic transcription of the word. Both dictionaries also indicate whether a word is among the most commonly used in English.