Energy Efficient and Cheap Servers for Home Use?

CapnRob asks: "I just got married, and my wife and I are putting together a home network in the (small) apartment we're now living in. We'd like to set up a firewall/mail server/small-file-server, but all the machines we own right now are pretty big machines that pull a fair amount of power, and that we don't want to keep running 24/7. Since our mail and file server needs are pretty low, our ideal box would be something like a Linksys WRT45G with one of the open source firmwares ... if only you could add a small hard drive to it. We're both long-time FreeBSD users, so installing a *nix system is no big deal, but what I've found so far in this line needs more l337 soldering iron skillz than I've got. Any suggestions for tiny little cheap boxes that won't send our power bills into orbit?"