Arduino or the Raspberry Pi.
The routers arrived on Friday so today I flashed OpenWRT onto one of the new router. To my surprise, the latest version of OpenWRT now comes with the web interface (LuCI), which means no installing it from the command line. That's really convenient if I ever want to configure something that way.
Once flashed, the router needed to be configured as a wireless client. That means connecting to the wifi in the house and passing that connection through to the ethernet port. Luckily, thanks to an awesome Instructables page, I already have one of these routers connected to the Snail Mail Notifier so the setup was pretty easy. I just grabbed the config files from the existing router, changed a few static IP addresses, and I was up and running in a matter of minutes. I can't tell you how much easier that was the second time around.
So we now have a Raspberry Pi on the wifi. The setup may not be as pretty as this miniature wifi dongle, but I'm happy with it.