Readability is available for most major browsers—Readability 2.0 is not free, but does offer more options than the free version. The free version is available as an extension for:

Readability can also be installed in any major browser—Firefox, Opera, Chrome, Safari—as a Bookmarklet.

Note: Readablity comes as a core component in Safari called Reader—the Reader button appears at the right end of the Safari address field. However, Reader for Safari is available only for Mac OS X v10.5 Leopard or later, and only appears when a webpage contains text-based articles. Based on my tests of Safari and Reader, Safari is fairly stingy about when it decides there is enough text to warrant turning on Reader. I recommend installing the Bookmarklet so you can turn Readability on when you want to use Readability, not when Safari decides you should.