Mods: "Lifeblood of Gaming Industry"?

Jadsky writes "Salon is is running a story about how modifications to games are now the lifeblood of the industry. It cites "Day of Defeat", an add-on to Half-Life, and proceeds to give an analysis of the history and current work on game mods. It also mentions Castle Smurfenstein and the Doom Construction Kit, which many of us played with before there was z-space."