Skype (WinRT) 1.6
I've been pretty critical about the horrible Skype apps in the past, but I have to say that tonight's Skype update (version 1.6) and the recent Windows Phone update actually seem to have them working really stably finally. I am now actually able to close the app and get chat and call notifications on the WinRT-based app, and the same has gone for the WP8 version for the past week or so. Also, like the WP8 version, the WinRT version has just added HD video support.
So if you haven't yet, you might want to update it and give it a try. I don't know for sure that the app will continue to work as well as it has so far, but I'm pretty impressed at the moment.