A server in a plug top format (10.5 cm (H) x 7 cm (W) x 4cm ( D)) at 2W or so for the 1.2 GHz CPU. 1 x SD slot, 1 x USB, 1 x GB Ethernet. 512M of flash memory on board. No WiFi on board, though it will presumably take a USB dongle without a problem.
The snags for the maintream: it only runs Linux: it runs on an ARM processor and this is, in essence, a developer's board (the Sheevaplug from Marvell). It runs Ubuntu immediately on boot up, so will run any other Linux with an armel port.
Portable webserver for conferences / advertising? Telemetry / remote sensing data monitor? Home heating controller? Ham radio rotator controller + data terminal equipment?