Readability is a "simple tool that makes reading on the web more enjoyable by removing the clutter around what you're reading."
This is absolutely brilliant, and something I'll likely use until the end of time. It's similar to Shaun Inman's Fix Width bookmarklet, but it goes a few steps further by stripping all of the crap surrounding the content you actually want to read, and giving you various options as to how you want that content displayed (e.g., font size, sentence length, etc.).
I've thrown at least 30 different sites at it already and only once has it rendered improperly (in which case you just reload the page and read it as it was originally presented).
It's hard to over-emphasize how nice it is to have a consistent feel to everything you read on the web, especially long-form stuff. (While on the subject, it might make sense for Live Ink (see my write-up) to make use of Readability so that the only thing the user has to do to get Live Ink's restructured text — within the browser window — is click the bookmarklet.)