Smallest IP Target Device?

Donimo asks: "I have seen many tiny ethernet loopback jacks and keyfobs, and I'm looking for the smallest IP equivalent for my toolkit. It just needs to plug into an RJ45 ethernet port and respond to ping. If the IP address is configurable, so much the better. I have looked at small IP print servers, and I could probably knock one up from a Gumstix but these are comparatively expensive solutions. Any ideas?"