I've got a couple of Pi's that are used for various things including:

• Magazine production - all of the DTP / Layouts and graphics for The MagPi Magazine are done on a Pi
• Media center
• Humidor temperature and humidty monitoring / control using sensors from Yoctopuce
• Interfacing with an Arduino

My next project is to replace my old garage door remotes by connecting a Pi to the door control within the garage, then the door can be operated from any phone attached to the house wifi network.