I started using an Arduino microcontroller board and a SheevaPlug together, and I'm very happy with the combo. The Arduino provides low-level analog and digital I/O, and the SheevaPlug provides computation, storage, and web service. All I needed to do was install a kernel with usb serial support, and then connect the two with a USB cable.
More details are on my blog at
http://arcfn.com/2009/06/arduino-sheevaplug-cool-hardware.html
