What is a service where I can perform side by side comparison of all fonts? Not just fonts on my computer, as most services seem to offer.
Myfonts.com is my usual go-to resource. They have an extremely broad library, a good system for type specimens, and a lightbox feature for tossing sets together.
Well I think it's not going to be possible to get one site that lists all fonts ever created but services like fontdeck.com, typekit.com, fontsquirrel.com, etc. often have ways to compare and contrast all the fonts that are available on their sites. All three of the examples I just listed do have quite a few fonts available as well so hopefully that'll be what you're looking for.
I just saw that Typecast is now out of private beta. It's a very slick tool for testing webfonts on screen -- testing them in your favorite layout app is a waste of time.