The Linksys NSLU2 is a powerful little device that is normally just used as an appliance to hook up USB drives to your network. In fact, these little devices run Linux and with the right amount of tweaking can run virtually any Linux application (such as Asterisk to make it a PABX, Apache to make it into a web server, etc.)

I have written a couple of guides on this topic:
General - http://www.ambor.com/public/nslu2hack/nslu2hack.html
Asterisk - http://www.ambor.com/public/home_pabx/home_pabx.html