This is a script for Greasemonkey for Firefox that modifies the Google search page to make the results show up as you type (not to be confused with Google suggest (greasemonkey script here), which guesses what you're searching for using auto-complete, based on the most popular search terms.)
Basically, it's like hitting "enter" after every character you type in the search box.

Link: erik's weblog Google Live Search
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Also, here's a new gmail script I just ran across, via new-links.