The Wonen app is a Windows 8.1 and Windows Phone 8.1 app that started as a project at school. I had to make an app for Windows Phone 8.1 that could show all houses in The Netherlands that are for rent. I have used an API to get this information. Later on the app also showed houses that are for sale.
Because the app used the WinRT platform it was very easy to also convert it to a Windows 8.1 app. This means almost all code (except for the XAML for the views) are shared across both platforms. Even the settings and favourite houses are shard across all devices thanks to the roaming app data. Both platforms support live tiles and push notifications for when new houses are added to the API.