Stomped has posted an interesting article that looks back at the various commercial add-ons for the Quake titles. The article looks at the official, the unofficial, and the really really unofficial add-ons, and there are quotes in here from id Software's Todd Hollenshead, Levelord (who worked on Scourge of Armagon, the first add-on for Quake 1), Rogue's Jim Molinets and others.