After spending a few hours trying to discover why my standard WordPress installation wasn’t generating a valid RSS feed, I tried this site. I put in a few good guesses as to the proper URL. After my third try, FVO auto-filled my URL and got it right!
So if you can’t get your RSS feed working, try this site.
(I installed a slew of RSS plugins and widgets fixing and verifying my feed, but still didn’t know the correct URL. No, it wasn’t thissite.net/wordpress/rss/ !!)