When I was a bit younger I used to run NT4/Server 2000 with a domain and active directory at home! Overkill doesn't really cut it, but was a great way to learn.
Much more simple now:
Asus RT-N66U Gigabit Router
BT Openzone Modem (80Mbps down / 20Mbps up)
HP Microserver N40L, modded with 6x...