OS X has a neat text-to-speech engine that'll read back what you write. You can start and stop TTS from the contextual menu or launch it using a keystroke that you set up in the Speech section of the System Preferences. Most of the settings for TTS are buried in the System Preferences which is inconvenient when you want to change a setting on the fly. If you need more flexibility than what OS X offers, you should take a look at Text2Speech. Text2Speech is a no-frills utility that uses OS X's underlying engine to read your text back to you.
The app gives you fine control over OS X's TTS engine in an easy-to-use UI. Once you paste your text into the app, you can change the voice that's speaking, change the speaking rate in small increments, and toggle the speech on and off with ease. It also tells you the character count of the passage, which is useful if you're writing a paragraph for a character-limited text box.
Text2Speech works well, with one caveat. When you start the TTS, it always starts at the beginning which is a minor annoyance. It would be useful if the app would let you choose the starting position. It would also be helpful if it remembered your position when you stop it in mid-passage. Despite these drawbacks, I still use Text2Speech every day. I find the convenience of being able to change settings on the fly outweighs these detractors.
If you want to try it yourself, Text2Speech is available for free in the Mac App Store. There's also a Pro version for US$3.99 that'll export your text to iTunes as an audio track or to your drive as an MP3 or AIFF file.