Although Xbox Live is no longer available on the original Xbox, system link can still be enjoyed. There are 136 system-link games on the original Xbox to play multiplayer, full list below. Naturally to system link Xbox games each console connected (LAN) must have a copy of the game. However, with a soft-modded Xbox you can backup the game to each Xbox console.
The original Microsoft Xbox and Xbox 360 can both be system linked to play (LAN) games. How to System Link your Xbox.
Also look into XLink Kai, the program that lets you connect with other console users around the world, and play online games for free. XLink Kai tricks your console into thinking that the other users it is connecting to over the Internet, are actually part of a Local Area Network (LAN).
Learn More about XLink Kai
XLink Kai Official Website
Download XLink Kai for Windows, Mac OS X or Linux
XLink Kai Forums for Original Xbox
Tutorial: How to Setup XLink Kai for Free Online Gaming (System-Link)
List or System-Link Games for the Original Xbox (Learn More)
187 Ride or Die
25 to Life
AFL Live Premiership Edition
Amped 2
Black Hawk Down
Blazing Angels: Squadrons of WWII
Brothers in Arms
Brothers in Arms: Earned in Blood
Brute Force
Call of Duty: Finest Hour
Call of Duty 2: Big Red One
Call of Duty 3
Carve
Chicago Enforcer
Close Combat
Colin McRae Rally 2005
Commandos: Strike Force
Combat Task Force 121
Conan: The Dark Axe
Conker: Live and Reloaded
Conflict Global Terror
Corvette: Zero to Gone
Counter-Strike
Crash Tag Team Racing
Crimson Skies: High Road to Revenge
Dead Man's Hand
Deathrow
Doom 3
Doom 3: Resurrection of Evil
Far Cry Instincts
Far Cry Instincts Evolution
Fight Club
Flat Out
Ford vs Chevy
Forza Motorsport
Full Spectrum Warrior: Ten Hammers
Godzilla: Save the Earth
GoldenEye: Rogue Agent
Greg Hastings Tournament Paintball
Greg Hastings Tournament Paintball MAX'D
Gungriffon: Allied Strike
Halo
Halo 2
Heroes of the Pacific
Hot Wheels: Stunt Track Challenge
Hunted: The Demon's Forge
Indy Car Series 05
Inside Pitch 2003
Iron Phoenix
Jedi Academy
Juiced
Land of the Dead
Links 2004
MechAssault
MechAssault 2
Men of Valor
Midnight Club 2
Midnight Club 3
Midtown Madness 3
MotoGP: Ultimate Racing Technology
MotoGP 2
MotoGP 3
MTX: Mototrax
NASCAR Heat 2002
NBA Inside Drive 2004
NFL Fever 2003
NFL Fever 2004
NHL 2K3
NHL Rivals 2004
Operation Flashpoint: Elite
OutRun 2
OutRun 2006 Coast to Coast
PainKiller: Hell Wars
Pariah
Panzer Elite Action: Fields of Glory
Phantom Dust
Powerdrome
Project Gotham Racing 2
Project Snowblind
Rallisport Challenge 2
Raze's Hell
Return to Castle Wolfenstein
Robotech: Invasion
Rogue Trooper
Sega GT Online
Serious Sam
Serious Sam 2
Shadow Ops: Red Mercury
Shattered Union
Splat Magazine Renegade Paintball
Soldier of Fortune 2
Spikeout: Battle Street
Spy vs. Spy
Star Wars: Battlefront
Star Wars: Battlefront 2
Star Wars: Republic Commando
Star Wars: The Clone Wars
Steel Battalion: Line of Contact
Street Racing Syndicate
SWAT: Global Strike Team (box claims support but apparently it doesn't work)
Timesplitters 2
Timesplitters: Future Perfect
TOCA Race Driver
Tom Clancy's Classic Trilogy
Tom Clancy's Ghost Recon
Tom Clancy's Ghost Recon: Island Thunder
Tom Clancy's Ghost Recon 2
Tom Clancy's Ghost Recon 2: Summit Strike
Tom Clancy's Rainbow Six 3
Tom Clancy's Rainbow Six 3: Black Arrow
Tom Clancy's Rainbow Six: Lockdown
Tom Clancy's Splinter Cell: Chaos Theory
Tom Clancy's Splinter Cell: Pandora Tomorrow
Tom Clancy's Splinter Cell: Double Agent
Tony Hawk's Pro Skater 2x
Tony Hawk's Pro Skater 3
Tony Hawk's Pro Skater 4
Tony Hawk's Underground
Top Spin
Tron 2.0: Killer App
Unreal II: The Awakening
Unreal Championship
Unreal Championship 2
V8 Supercars 2
V8 Supercars 3
Warpath
Whacked!
World Series of Poker
World War II Combat: Road to Berlin
World War II Combat: Iwo Jima
XIII
Yu-Gi-Oh! Dawn of Destiny