Despite many saying that this is something specific to Call Of Duty 4 it is in fact a general term for any weapon that is cheap and/or over-powered in any video or computer game, usually a grenade launcher or RPG but also, especially with newer games, can be a shotgun (i.e. Call Of Duty, Half-Life 2, Rainbow 6) at least when up close. Use of it is commonly frowned upon unless used with some finesse or tactical forethought.
Tom: "Dude, I was playing Halo 3 the other day some dude unloaded on me with the Mini-Gun."
Harry: " The Mini-Gun is such a Noob Tube"
Dick: "Gosh that guy just shot me with a Noob Tube."
Tom: "What a Noobish thing to do!"