Their entire company owes its history to mods.

HL's engine GoldDrc was originally a mod for Quake. Team Fortress Classic was based on a quake mod. Counterstrike was a HL mod they bought out. Portal was a student game they bought. Dota 2 was based on a WC3 map. Left 4 Dead was a mod made by Turtle Rock while working on CS:CZ (so, yet again a mod, although a mod based on their own engine this time and build in house). Underlords was based on a Dota 2 mod.

Deadlock is original, but based on characters and lore from the game they made from the WC3 map.

Deadlock and L4D are arguably the only true original creations.

Valve knows their bread is buttered by outside creation using tools and platforms they can provide and then fold in if it catches their attention.