Firefox is known for having its inline spell checker that underlines misspelled words as you type them. I truly believe that this feature in Firefox has helped the Internet as a whole become more readable by reducing the number of misspellings in even the simplest things, like comments.

This is actually "old" news, but worth pointing out.  Back in December, Google added their first bit of Ajax to their search.  Although it's been around for a couple of months, it took Read/Write Web pointing it out before I even noticed it.  Sometimes these new features blend right in, and it takes pointing it out before you notice it.