Mac OS X Mountain Lion added built-in support for voice dictation. To activate, insert your cursor where you want to dictate and double tap the FN key on your keyboard. Tap the FN key again to finish.
If the microphone icon doesn’t pop up you may need to enable the feature in the Apple menu > System Preferences > Dictation & Speech > Dictation.
While it’s not as robust as fully featured dictation software like Dragon, it works pretty well in a pinch. The biggest drawback is that most dictation software will type while you talk, but Mountain Lion’s dictation only transcribes after you finish. You will also need to have a live internet connection.