Alltheweb Dynamic Search - New SEO Tool?
 

http://livesearch.alltheweb.com/

Keyword suggestions, plus dynamic results as you refine your query. Yahoo owns Alltheweb, so ads you see and search results are similar to what you'd see on Yahoo.

EDIT: More details, raves and rants on Yahoo! Search Blog

http://www.ysearchblog.com/archives/000301.html