GhostReader allows you to listen while your Mac does the talking in your language of choice.

Why buy GhostReader if Mac OS X includes free text-to-speech?

Sure there are text-to-speech functions built into Mac OS X, but they do not do everything GhostReader does, do some things in less practical ways, and, unless you buy third-party voices, won't speak any other language than American English. Just try letting Preview read a PDF to you and you will find that you need to go into a hierarchical menu to start and stop reading. Try letting the reading start in the middle of the page: impossible. Try to convert the PDF to an iTunes track: impossible. Now open the same PDF with GhostReader. It will extract the text and will let you put the insertion point anywhere you like so that you can listen from that point forward. You can fast-forward, rewind, skip paragraphs, export as audio to iTunes. Now, which is more practical to listen to PDFs, Preview or GhostReader.

Now turn on speak text below the cursor in the GhostReader preferences and set it so that it will only speak when the option key is held down. Now hold your option key down any time you want to listen to the paragraph below the cursor in Safari or Mail. Alternatively, select any text and press the speak selection button. Compare these ways of reading text with that of the Mac OS X function that will speak selected text with a hotkey. Which is easier?

People talk a lot about VoiceOver. VoiceOver is great solution and provides excellent access for blind users, but would you use it just to have it read some text to you occasionally? It is not designed for that purpose, requires lots of keyboard commands, speaks all the time...