I have also come across webreader.readspeaker.com/ installed on the website that will read out your website to people which is cool
This type of service I'm starting to see on more and more newspaper/article sites. It's not for the totally blind (who are very likely to have their own screen reader/refreshable Braille device, otherwise how are they using a computer on their own?) but I suppose an assistance for everyone else: low/degrading vision, dyslexics, or those who learned the site's language as a second/third/etc language and maybe speak/listen better than they read.
A screen reader is much more, though. It's got a whole set of navigation tools built in and it understands markup (among other things). It can be hooked up with a screen magnifiuer for low-vision folks or to a refreshable Braille reader for those who've learned Braille (and even at 16 or 32 characters at a time, people using those are way faster than people with just a screen reader... plus lack of speakers isn't an issue then : ) It's not just passive reads-stuff-to-you software.
NVDA is great for testing websites. It seems to be better at forms, tables and ARIA-stuff than JAWS, tho I don't have the latest JAWS anymore so who knows. Screen readers are like browsers: they have their own bugs and quirks. Just because one reader does something correctly doesn't mean all the others do. Using one is good for getting a feel for your website though, in a way using Lynx doesn't.
Oh and since a screen reader reads through a browser, you might notice slight differences between browsers as well. You can run NVDA on Firefox and IE, though I dunno if it works on Chrome yet (didn't used to, JAWS neither) since Chrome hadn't implemented its accessibility layer (where information from the page code is offered to the screen reader)... and Opera, I've never dared : )