1.True Crime: LA: Free mode
2.CS:S: Free steam keys
3.L4D: Free steam keys
4.GMOD: Free steam keys
5. Blockland: 4 Free! Well i really like blockland but it costs like CS:S....Well maybe not free but 5-10$
Wow you can't even afford those and want them free...
Ontopic: L4D, not really a game adding onto, but my favorite game that would have to be made would have-
-Zombies
-Mutated things
-First Person Shooter
-Tons of interactions (Kicking down doors, hand-to-hand combat, turning lights off)
-Multiplayer FTW
-Thousands of weapons, gear, etc.
-Vehicles
-HUGE MAP
-Looting and Missions
-Humans still "Selling" stuff and trading for things you might have
-Bosses
-Gear like Boots, Vests, Helmets, gloves.
-Only carry depending on appearal such as- Vests can carry more magazines/flashlights and stuff, while a shirt cannot carry anything and jeans can only carry a tiny bit in pockets
-HUGE HORDES OF ZOMBIES
-Safe house with boxes that can store the stuff no equiped/ in inventory for more room
-Safehouse can hold 1 vehicle that may hold mutiple other people and you can do looting in it and stuff
Yes, as you can see, pretty much an extremely real Zombie Outbreak game. <3
(P.S, add onto if you guys want hehe)