Short version blurb about search engines: google's an index, yahoo is a directory. Go to google when you want to learn more about a subject, yahoo when you want to find specific information on something you already know about.
Today a 3rd contender has launched: cuil (pronounced "cool"... thank you web 2.0 phonetics). They're deal is that they index, too, but the results are presented in a "magazine" layout. The page is meant to be read left-to-right instead of top-to-bottom, and they wedge in images from the sites that the blurb they show you came from.
From my quick survey of it, it looks interesting and has the potential to fill in the 'research' niche. Let's you quickly browse through pages and figure out what's relevant and what isn't. Also, they're supposedly focusing more on the content of the page instead of page rank for their search results, so the idea is that you're more likely to get pages about the subject your looking for rather than 50 copies of some SEO's honeytrap site designed around getting you to click on a banner ad.
Cuil's also saying they won't be tracking user histories, which raises the question of "just how do they expect to monetize and subsequently profit of their search engine?"
Will be interesting to see how it works out. Don't think it'll replace google, but it's nice to have some options.