Mac Dictionary, built-in to Mac OS X is a very capable and flexible Dictionary application. You can find it in your Applications folder. It has much more functionality than a simple ‘look-up a word’ style dictionary though… and like many areas of your Mac OS X operating system has some clever ways to conduct dictionary and thesaurus searches and even access Wikipedia!
Perhaps your elementary school teachers taught you how to decipher the pronunciation symbols provided by the dictionary, but…perhaps not. Did you know that you can get Dictionary to pronounce words for you?
Go to System Preferences, click Dictation & Speech, click the Text to Speech tab, and choose a voice.
To make Dictionary talk, select a word and Control-click (or right-click) it. From the contextual menu, choose Speech > Start Speaking.