Media streaming, network backups, assorted virtual machines (web, Minecraft, cloud storage): 3.7GHz hexacore Intel i5, 64GB RAM, Windows
Quake, Ventrilo, QTSS: PowerMacintosh G3 / 350MHz, 896MB RAM, Mac OSX Server (Panther)
NG Firewall/Router: Custom built 3.4GHz quadcore Intel i7, 24GB RAM, Untangle OS

These systems are currently in full-time operation. I've done what I can with what is essentially a mobile setup.
This is somewhat outdated. Specs are accurate, but I'm in a house and use a simple wire frame "rack". Server racks can be a mega PITA (finding proper rails for 4 post setups to use with these machines is annoying).