Honestly, everyone uses Doom 3 as the ultimate benchmark/productivity killer of all time. But, have we forgotten our roots? The Classic DOOM games are availible at most retailers for $10. There are enough ports to make you drop a load. Most notably is ZDaemon, a port that allows you to play Classic Doom online with other people. Game modes include DeathMatch, Team DM, CTF, Co-Op and Rocket Jump (Try it, it's fun

) If you're looking for something to do on your attic-ware, this it worth it.
ZDaemon.org