Bert Lagaisse from Virtual Remote has a couple of nice apps, this one I like particularly because it is so practical for us that are regular presenters. This little app turns your Windows Phone into a very fancy Remote Control for your PowerPoint presentations. You have to install a small companion app on the PC side to allow connections and setup the handshake (including security) and then you can connect over Bluetooth or WiFi.
The app works 2 way and displays your current PC screen allowing Primary or Second screen. Good help included.
If you are using Windows Media Center functionality there is a version for that too: http://www.virtual-remote.com/products/mediacontrol