I made a simple concept design for an HipChat Windows Phoneapplication, that currently is missing on the Marketplace. There are a lot of Jabber clients, but none of them is slick such as an official app could be. Have a look at these shots and some more on my Windows Phone projects page on dribbble.
From design to real app
Shortly after designing it, I've created a prototype for Windows Phone using C# and a native Jabber library in just a few weeks. The app got so much interest from the Windows Phone community that Atlassian contacted me about funding the development of the app, since they didn't have enough resources to make their client for such platform.
You can read more about the project or simply contribute by looking at its open-source repository on Bitbucket.