The Yahoo! app for the iPhone is in the News category in iTunes, which is a bit odd, since the app does so many things. Most recently the company added voice search to its app, so that you can speak your queries. (Voice search has been available in Google Mobile App for some time.) In addition to offering search, Yahoo's app offers news, e-mail (Yahoo, Google, Hotmail, or AOL), status updates from Facebook, Twitter, and other social networks, RSS feeds, Yahoo Calendar, Address Book, and Messenger — and much more.
It's quite impressive. You can read a short review on MobileCrunch that compares the two apps.

Yahoo vs. Google
Jim -- Sounds interesting. I see Yahoo all day as I access my e-mail. How does their voice search work compared to Google's app?
I haven't had success every time I've tried to show that Google app to others. Is it my pronunciation, or enunciation?
And who do you think is going to win this voice search game: Goog or Yahoo?
Hi, Tim. My experience has
Hi, Tim. My experience has been the same as yours: voice recognition doesn't work that well. I've only tried Google's but doubt that Yahoo's is better. Still, I get excited about this because it works well enough that I see the potential. I think these apps will just keep improving, and I'm betting on Google. They always seem to get things right.