A good old PC with a flash card.
You could then install :
- PFSense
- M0n0Wall
- IPCop <- Not sure if still exist...
- Linux with iptables.
- Tomato
What I did in my case, I purchasses a Soekris device. They have plenty of differents devices. You can check here : http://soekris.com
I personally run PFSense. It's based on a BSD, same as m0n0wall.