The intrusion.com PDS 2100 series of appliances make an excellent and
affordable m0n0 platform.
There are several on ebay and I just purchased 2 for $9.95 each.
The one gotcha with these devices is that they don't have video cards (they
use a serial port instead) so you have to connect a PC to the LAN interface
and use a web browser to configure them. Or you can customize the kernel to
use the serial console.
Most come with 3 10/100 ports, 2 USB ports, Celeron 600 CPU and 64 or 128
megs of ram (can replace the DIMM to increase size).
They have a CF slot for booting and some include 10 meg SFF (laptop sized)
Vince Van De Coevering
Figaro's Italian Pizza, Inc.
vpv at figaros dot com