The Raspberry Pi project is amazing. I recently order a few to try out, and I am going to post pictures and descriptions of some the projects I implement.
I have written a bit about running MODBUS on a Raspberry Pi here.
For home security I have written a python script do implement simple motion detection here.
I recently purchased a bunch of DC motor controllers and an Arduino UNO for my Raspberry PI with the goal of creating a robotics platform for mowing my lawn! I hate cutting the grass, maybe my Pi will like it more than me? Click here for the RaspiMower project.