If LFS was gone, I'd be playing alot more SMOD:Tactical and Insurgency:MIC
Smod:Tactical is one of the hardest single player FPS games ever created. It requires alot of time to come up with a strategy to survive just ONE firefight. The game has you thinking constantly. One bullet can kill you, and your enemys are as skilled as US navy seals. The only benefit for you is that the enemy is still human, and one bullet to the chest ususaly puts them down as well as it would you.
Insurgency is similar but multi-player, so you get crazy firefights with real people, and unlike counter-strike it takes realistic strategy to win. There's no run and gun, no bunny hopping and no random headshots because the computer decided the bullet just happend to go that way. It takes actualy putting the sights on the enemy to hit them instead of dumb luck with a firing cone.
Both of those are source modifications, I don't play "Games" that much anymore because FPS developers seem to have lost the spark they had 10 years ago. There are only a fue modern FPS games I like. Other than that I rely on Mods for interesting and unique gameplay.