All Valve games ever, full stop.
Why?
Free DLC, free updates, engine improvements, and tons of custom maps, mods, and campaigns. The only game this doesn't apply to is Portal, and even that had Portal: Prelude.
Why?
Free DLC, free updates, engine improvements, and tons of custom maps, mods, and campaigns. The only game this doesn't apply to is Portal, and even that had Portal: Prelude.