Hi all,
http://www.linuxpromagazine.com/online/ … pen_source - Yoggie released their Linux ARM-based (XScale) hardware platform with full Linux shell access, along with SDK and BSP source codes. Meaning, you get the full PXA270 @ 520MHz, 128MB SDRAM, 128MB NAND Flash, USB product, platform and everything for $49 (!!!)
As far as I know this is BY FAR the cheapest, strongest, smallest platform available… 
The device already comes with firewall application running, with GUI and everything. Beyond the fact you can play around and research what’s going on inside or even implement new features – You can actually use this as a platform to develop anything you want from scratch.