Corpus is Latin meaning "body" and a corpus is a very large body of text. A corpus won't provide pronunciations, but it is a great way to find hundreds of example sentences of words. I recommend this version of the
British National Corpus because it's simpler. Just type a word into the "Look up:" box and you will see 50 random examples of that word used in real life. For example, possibility and opportunity are very similar in the dictionary but if you compare example sentences from a corpus you will start to see that they are used a little differently. A corpus is also a good place to find what prepositions are used with certain verbs, nouns or adjectives (do we say responsible of/for/with something?)*.
For pronunciation you can try the
Macmillan dictionary or
How J Say.
*responsible for something ;-)