Nest Thermostat is an unofficial third party app for controlling a Nest Learning Thermostat from your Windows Phone 8 device. Current features of the app include display of current temperature and humidity, adjustment of temperature, and the ability to toggle home/away and fan on/off settings.

The Nest Learning Thermostat is an electronic, programmable, and self-learning WiFi enabled thermostats that optimises the heating and cooling of buildings, thus minimising energy usage. It is compatible with most standard heating, ventilation, and air conditioning  systems, effectively acting as a remotely accessible, smart control point, for heating and cooling appliances. 

For remote access Nest allows you to control your thermostat through a website and there are also Nest Mobile apps for iOS and Android. With this unofficial Nest app there's now also a native solution for Windows Phone. The Android and iOS apps do have additional features, such as the ability to view your energy history, but for most use cases the basic controls available in the Windows Phone app should be sufficient.

Windows Phone Store description:

The Nest Learning Thermostat, created by Nest Labs, is one of the smartest and most innovative thermostats you can get for your home.

With this app, you can now view and control your Nest Thermostat anywhere from your Windows Phone!

The Nest Thermostat app can be downloaded from the Windows Phone Store.

Here's a video explaining the Nest Learning Thermostat in more detail: