Dragon Dictate (formerly Mac Speech) is a professional dictation software that allows you interact with your Mac using only your voice.
Dragon Dictate has been designed specifically for Mac OS X and helps you create reports, emails, articles, books, research notes, online content - just about anything you'd normally use a keyboard for. The developer claims it has 99% accuracy, although you'll definitely find it doesn't quite measure up to that claim. However, the overall reproduction and accuracy is generally very good.
Dragon Dictate can even be used to command your Mac to carry out certain functions such as “Reply To This Message” when using email, or “Open Microsoft Word” or “Search Google for software” etc. You can also customize Dragon Dictate with personal vocabulary and voice commands for specific tasks related to your work.
A lot will depend on the kind of microphone you are using. Although it works with any microphone, the developer also provides its own when you purchase the full version. It does work with the Mac microphone very well, although it's subject to instability from time to time.
Dragon Dictate could increase your productivity significantly - its at least worth a try.