Old laptops also make killer home routers. I recently replaced my consumer grade wireless router with an old Thinkpad in a router-on-a-stick configuration + PoE switch + dedicated access point and my home network is better for it in just about every way. Added bonus that it can run a simple webserver and run some light home automation tasks without breaking a sweat.

Of course the downside is that you lose a nice web GUI for managing port forwarding and stuff, but if you're SSHing in to adjust iptables on your router you probably don't need the web interface all that much anyway.

It's probably true that an off-the-shelf router uses less power, but if you already have an old laptop lying around, at what point do you break even on the electricity cost vs. buying more efficient hardware? Especially when the difference is on the order of a few watts.