Yahoo has introduced "Yahoo Search Direct", which seems to be similar to "Google Instant". It is incorporated into Yahoo's web search page.
http://search.yahoo.com/

It can be enabled or disabled from the Yahoo Search preferences page, which has a setting for "Search Direct" and a setting for "Search Pad".
http://search.yahoo.com/preferences/

Searchengineland reported on their comparing Yahoo Direct to Google Instant with some sample terms.
http://searchengineland.com/head-to-head-yahoo-search-direct-vs-google-instant-69681

http://www.eweek.com/c/a/Search-Engines/Yahoo-Search-Direct-Challenges-Google-Instant-436862/
http://news.cnet.com/8301-13577_3-20046240-36.html

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http://ycorpblog.com/2011/03/23/searchdirect/