If you got a network, you probably want to play games together. But you don't want to spend money!!
So I made a little list of multiplayer shareware. All these demo's (or freeware) are capable of LAN-multiplayer. I tested them myself.
Game OS
Codename: Eagle Windows
Daikatana Windows
Doom Windows Unix
Mechwarrior III Windows
Parsec Windows Unix
Soldat Windows
TankTime 3D Windows
Transport Tycoon Windows
Xblast Unix
Xevil Windows Unix
XPilot Windows Unix

Have fun!