Another app is the HoloLens version of Skype:

"We believe Skype is an important experience to highlight for developers for two reasons. First, it shows a best in class example of how you can build a holographic application that can be enjoyed by people who don't have a HoloLens. Skype on HoloLens allows people running Skype on any Windows device to interact in the holographic world. This application will show how holograms can be used for remote collaboration and training in a way previously impossible. This type of communication has enormous potential in both the consumer and business world. Secondly, it will allow developers to communicate with each other using the holographic medium to share development methods and seek the advice of others. With Skype, you can see the holograms the other person is seeing and you can use holograms to illustrate helpful techniques or development approaches."