Fangs is a Firefox extension that emulates the output of a screen reader in plain text.
Why would you want to use this instead of an actual screen reader?
Screen readers by their very nature are linear. Testing sites with lots of content would take ages; you would need to listen to the linear version of each page, and it is difficult to skip and rewind. Fangs allows you to quickly scan through the output of what a person would likely hear from a screen reader and make adjustments to your client-side output accordingly.
Fang does not replace testing with the real screen reader products. It doesn't replicate the interaction a vision-challenged user would have with the website via a screen reader. For example, you can't navigate from page to page using Fangs. However, it does emulate the voice output, which will save you quite a bit of time when you're making small and iterative changes.
FYI — the default output style for Fangs renders the page in one continuous block of text. I recommend changing to the Sectioned output style.