Everyone talks about the symantic web, AJAX, Content Management Systems, Flash, and various other ways to fancy-up your website. And developers are told time and again about avoiding things that take away from the simple usability (and legibility) of the sites they create. But not everyone listens, and a significant proportion of the web is just illegible. These websites are designed for a different device, screen size, browser, or just not designed properly at all (animated background gifs, anyone?). So we end up trying to read things that are too small, too wide, illegible against the background or in a stupid font like Comic Sans.
So how about a browser extension that allowed you to choose to make that body of text legible? One that used all the well-researched legibility advice as defaults (alphabet and a half width, ragged right, larger fonts, better contrast) and allow users to tweak their settings.
The interface could be fairly simple - an inoffensive button similar to that in AdBlock, which would toggle the functionality for specific boxes, and the option to have it globally on. Functionality would revolve around undoing a lot of the things designers do (mess up fonts, colours, link underlining, silly backgrounds, etc) and set things that designers are unaware of (better line lengths of 30-50 characters, left justified text, paragraph indenting, greater leading, black on cream text).
Some users need even greater legibility through vision loss, so for them a mode with a different set of default values would be worthwhile. If everything is customisable, then few will find it doesn't help them in some way, but having the best set of defaults would be key.
Some pages that give legibility information: Web Style Guide, Wikipedia. As a side note, it seems that readability means something else - the complexity of the text itself, rather than its presentation.