My Yahoo! Search
It’s pretty cool. You can do the standard search stuff, but if you log in with your Yahoo account, it lets you do a bunch of other stuff as well — stuff like saving pages that you like from your results to a “My Web” storage area for easy reference, blocking pages that aren’t relevant from search results, making it easy to share interesting sites with friends by e-mail, and more. You can even use the My Web function much like the way people use del.icio.us, as a kind of public linkblog — it even publishes an RSS feed for your My Web content.
Of course, the key question is how good the search results are; I haven’t played with it enough to be able to say definitively if it is Good Enough yet. But if it is, this could definitely give Google something to stay up nights worrying about — it’s the first real innovation I’ve seen applied to the search experience since, well, the launch of Google.