Stonegate SG-200 Appliance
These little appliances went end of life a couple of years back, so you should be able to pick them up 2nd hand fairly cheaply.
They are a little red box, just slightly larger than the size of two DVD boxes.
They pack a Cyrix geode running at 300MHz with 128 MB RAM, so they're never going to win any performance awards. They do have three realtek 10/100 LAN ports and a serial console port on the back. There are also functioning network link and activity indicators at the front.
Storage comes in the form of an internal CF card slot - you'll need to get a new CF card as the one they come with is a little small.
And .. they can run pfSense 2.01 :)