Mac OS X includes several voices built-in, but they are somewhat limited, and only US English. If you'd like more voice talent, including different accents and languages, you can download and try the high quality voices provided by Cepstral LLC. Try them here! Type some text (or use the default), and click Action! to listen to the selected voice (it may take a few moments to be received by your computer):

(If for some reason it doesn't work, try Cepstral's demo page instead.)

If you like what you hear, click the Download Now button to get any number of voices from Cepstral's downloads page, or click Buy Now to purchase voices from their store.