I've been trying to find a list of games that support system link/LAN gameplay.  Sadly this list doesn't exist on the internet.  At least not a comprehensive list covering multiple systems.  If you know of a game that's not listed that does support System Link/LAN let me know in the comments!

I'll also be adding some links to some LAN related sites at the bottom of this page.

Playstation 2
Champions Return to Arms
SOCOM Combined Assault
The Lord of the Rings The Return of the King

Xbox
187 Ride or Die (2-4)
25 to Life (2-16)
Baldur's Gate Dark Alliance II (1-2 players)
Blazing Angels Squadrons of WWII (2-16)
Brothers In Arms Road to Hill 30 (2-4)
Brothers In Arms Earned in Blood (2-4)
Call of Duty Big Red One (2 - 16)
Call of Duty Finest Hour (2-16)
Counter Strike (2-16)
Crimson Skies High Road to Revenge (2-16)
Delta Force: Black Hawk Down ( 2-32)
Doom 3 (2 - 4)
Farcry Instincts (2-16)
Goldeneye Rogue Agent (2-8)
Halo (2-16 players)
Halo 2 (2-16 players)
Halo 2 Multiplayer Map Pack (2-16)
Heroes of the Pacific (2-8)
Mech Assault
Mech Assault 2 Lone Wolf (2-12) (HD 16:9)
Outrun 2 (2-8)
Pain Killer Hell Wars (2-16)
Pariah (2-16)
Project Gotham Racing 2 (2-8)
Return To Castle Wolfenstein: Tides Of War (2-16)
Robotech Invasion (2-8)
Roque Trooper (2-4)
Sega GT Online
Serious Sam
Serious Sam 2
Splat Renegade Paint Ball (2-16)
Star Wars Battlefront (2-24)
Star Wars Battlefront 2 (2-24 players)
Star Wars REpublic Commando
Time Splitters Future Perfect (2-16)
Time Splitters 2
Tom Clancy's Ghost Reacon 2 (2-16)
Tom Clancy's Ghost Recon 2 Sumit Strike (2-16)
Tom Clancy's Ghost Recon Advanced Warfighter (2-12)
Tom Clancy's Rainbox Six 3 )2-16)
Tom Clancy's Rainbow Six 3 Broken Arrow (2-16)
Tom Clancy's Splinter Cell Chaos Theory (2-4)
Tom Clancy's Splinter Cell Double Agent (2-4)
Tom Clancy's Splinter Cell Pandora Tomorrow (2-4)
Top Spin ( 2 or 4)
Tron 2.0 Killer App (2-16)
Unreal II The Awakening
Unreal Championship (2-4 players)
Unreal Championship 2 (2-8)


Gamecube
Mario Kart Double Dash

Xbox 360
Aliens Colonial Marines
Armored Core 4 (2-8)
Borderlands  (1-4 players)
Call of Duty World at War                                                                                    
Halo 3
Halo ODST
Halo Reach
Left 4 Dead ( 2-8)
Left 4 Dead 2 (2-8)
Red Dead Redemption
Resident Evil 5 (1-2 players)
Shadow Run (2-16)
Tenchu Z (1-2 players)


Playstation 3
Armored Core 4
Borderlands
Call of Duty World at War
Conflict Denied Ops
Fairy Tale Fights
Haze
Kane and Lynch 2 Dog Days
Lord of the Rings Conquest
Operation Flashpoint 2 Dragon Rising
Red Dead Redemption
Resistence Fall of Man
Saints Row 2
The Orange Box: Team Fortress 2
Tom Clancy's HAWX
Tom Clancy's HAWX 2
Tom Clancy's Raibow Six Vegas 2
Unereal Tournament 3
Warhawk

Links

Lanpartymap.com
Co-optimus.com
http://www.xbconnect.com/index.php?topic=supportedgames