Somebody mentioned this at work and I had to check it out.
https://addons.mozilla.org/en-US/firefox/addon/5761
It randomly snapshots any "text area" on a web page and remembers what you typed. So if you were working on a big blog post, answering some question on a web site, etc, and you accidentally navigate away, the computer crashes, whatever, you won't lose it ALL. A "text area" in HTML is anything more than a single line entry field.
