ISP supplied Wireless router, hooked up to a server (grandiose name for a Geode based 'computer' running Ubuntu) which acts as a secondary router and firewall. I used to use firewall distros but found them too limited when you wanted them to do something they didn't do out of the box. Like take pictures of your Elecricity and gas meters every 8 hours.
And keep an arduino on the straight and narrow :
http://pluggy.is-a-geek.com/arduino/index.html
I've used old desktop computers in my time as well, but the amount of electric they consume is depressing.........