The PS3 has mediocre LAN support, there are a bunch of good games that can benefit from it but developers won't take 12 hours to implement it. It'd be the easiest thing in the world for games that already have netcode.
I know Unreal Tournament 3, all CoD, Warhawk, FarCry 2, Red Faction Guerilla, RE5, Borderlands, Conflict Denied Ops, Fairytale Fights, Haze, Kane & Lynch 2, LotR Conquest, Operation Flashpoint, Red Dead Redemption, Saints Row 2, GRAW 2, Hawx, RSV2 (but not the original), Age of Booty, I think Medal of Honor..
Out of those not all games support LAN and splitscreen, some are 1 player per console.
Resistance, Fat Princess, Uncharted 2, Killzone 2, Bad Company 2, Aliens VS Predator, War for Cybertron, Modnation Racers, and Sonic & Sega All Stars could all use a LAN patch.