Venmo is a P2P payment service which allows users to easily transfer small amounts of money to friends and then transfer these sums to your bank account.
The service has apps for Android and iOS, and also has a public API to allow 3rd party developers to integrate payments into their apps.
Peter Battaglia, a Microsoft employee and also Windows Phone Store developer, has used the API to create a Windows Phone version of the app, which lets users make payments and charges, view their recent transactions, see your friends, and receive real-time notifications when you have activity on your Venmo account.
The Venmo service is US-only, and the app is currently Windows Phone 8.1 only, with a WP8 version on the way.
The free app can be found in the Windows Phone Store here.