I have a new link to an excellent Pronunciation Course from English-Online.  It has a series of graduated listening exercises to help English Language Learners distinguish between confusing letter sounds (p and d; s and sh, etc.).  I particularly like it because they demonstrate the differences in the context of a sentence in the last part of the exercise.

You can find the link at the bottom of the Phonics section (I label it “Pronunciation Game”)  of the English For Beginners page.