We often hear the most stories about all the cool multimedia entertainment devices running Linux, but here's a "real" embedded device without all the glitter: Embedian's EBC-series. There's no single purpose those have designed for, so you can pick your box and put into use. I can see various amounts of eg. industrial automation you could do with this device, and do it with a fast start - everything is ready for attaching eg. measurement devices. Also the tools for controlling eg. sensors, robotics or for analyzing data might also be readily available in some open source project already, or could be implemented on top of some more general open source solutions more easily than developing from scratch. The devices seem to have designed with heavy environment requirements in mind, so those are definitely not the choice for home environment, but offer quite nice amount of opportunities in industrial environments.
Of course, stuff like this has been available before, but combining Debian with low-power devices (max. 2W) like the EBC-series' models should give a good run for the money for any proprietary solutions with regards to flexibility and price.