FontFriend allows you to make typographic adjustments to text on the current page, such as changing the font, font sizing, font weights, line heights, text transforms, and font variants. You can even use jQuery selectors to interact with specific elements on the page.
ffffallback was created to help web font users find the ideal fallback fonts to use in your design by allowing you to see how they appear and compare on a page.
Min strips the page of colors, backgrounds, and borders, allowing you to focus simply on the typography of the site.
When used in conjunction with Lettering.JS, the Kern.JS bookmarklet allows you to easily make manual letter-spacing adjustments.
Curious about the fonts used on a particular page? Click the WhatFont bookmarklet and hover over some text to reveal the typeface.