InSearch is a Firefox extension that provides a keyboard shortcut to focus on the search box within the page being viewed.
How often have you been happily surfing a website from your keyboard when you want to carry out a search of items on the site, perhaps the forums? Although the search box is present on the webpage it requires you to use the mouse to select the search box before you can type in the text. With InSearch this is no longer a problem: on hitting the keyboard shortcut (shift-control k by default) InSearch will focus on a search box within the webpage, allowing you to search within the site without requiring you to use the mouse (or locate the search box on the screen).
InSearch is available now at http://www.devzero.net/firefox/insearch.xpi - give it a go and let me know what you think.