Yoggie Security Systems has launched open source models of their Pico and SOHO firewall products. The Pico is a USB key, which replaces OS network drivers and reroutes wired and wireless traffic through the device. The SOHO can be used in the same manner, but it can also be inline since it has two Ethernet ports. Both devices contain a 520MHz ARM CPU, 128MB RAM, and 128MB Flash.
The folks over at Linux devices goes over more features of the product, and they also have some screenshots of the web interface. Yoggie has the SDK for both products available on their Yoggie Developers page.