Firefox: You've just finished typing a well-reasoned comment explaining your take on an article on Lifehacker. You click submit and suddenly it's all gone. If you have the Textarea Cache extension for Firefox installed, getting it all back is as simple as right-clicking.
Textarea Cache is similar to previously mentioned Lazarus Form Recovery for Firefox, Chrome and Safari. It's not as robust, but its strength is in its simple operation and low resource consumption. It supports any text area, including Facebook comments, blog posts and comments, text fields on sites and surveys and more.
Best of all, open the options page to see a single-pane set of easy-to-understand options and stored text from sites you've recently visited. You can tell Textarea Cache to save text entered on all sites, or you can set up an exclusion list so it only stores text from specific sites that you care about. It's free and works everywhere Firefox does.