I couldn’t seem to find a good list to refer to, so I’m putting together my own. Here’s the three bookmarklets I’m finding to be indispensable when working with webfonts:
WhatFont
Ever wonder what that font is you’re looking at? This will tell you! (There’s also a Chrome extension if you’re into that.)
MIN
Absolutely awesome for examining hierarchy. It removes colour and decoration from the page so you can see what’s going on with your type.
FFFFallback
This one makes it a million times easier to decide on fallback options for your webfonts and actually know what they’re going to look like. It also uses “magical unicorns and javascript rainbows”, so you know it has to be good.