Ron Thomson at Brock U has put together a website to help students learn to distinguish between English vowels and consonants. It's currently in Beta, and will be changing a bit in the next few weeks, but it looks quite useful. It's also likely good for TESL students learning about phonology and pronunciation.
It's a game-like interface that plays syllables or words containing target phonemes that have to then be identified. The sound quality is high and the the pronunciations are varied but natural. I would strongly recommend using earphones rather than speakers.