What apps? I didnt know Yahoo even made stand alone apps for mobile.
Then there's the Yahoo owned services like Flickr, if you want to count them.
Edit: I just installed Yahoo Weather, and it's great. It uses photos to represent weather conditions at each of your saved locations, which are all pulled from Flickr. I doubt it has photos specific to every weather condition in every location, but I got them for all the locations I tried. And the animation in the interface (parallax scrolling between locations, spinning windmills for windspeed, and maybe more I didn't notice) is smooth and well done.
Pulling up a weather app and seeing a local landmark with a cloudy sky in the background is a really neat user experience, so props to Yahoo on successfully integrating Flickr with this. I like it a lot.