Prototype – jQuery LiveSearch (QuickSilver-Like)
Live search is a great way of improving usability, specially if you have a serious amount of data to search. No going back & further and your users can see the results as they type.
QuickSilver has a great interface which accepts abbreviations as the input like writing wrdprss rather than WordPress.
John Resig (creator of jQuery) played with it more and re-ported it to jQuery (which seems to be the final version for now : ) ).
Update (07.08.2008): And a MooTools version of the code.