Howjsay provides a list of around 150,000 English words that can be heard in a clear standard pronunciation. Although it includes quite a lot of publicity, the search is easy and quick and the quality of the sound good enough to be used in the classroom.
When the result of a search appears in pink, the word can be heard once and again just by placing the mouse over it. Furthermore, it allows to list up to 15 entries by writing them separated by semicolons: a very useful option for practising aural recognition of close phonemes.