Duct Tape is a Doom 3 mod and a add-on by Glen "FrenZon" Murphy. The first version was released in August 6, 2004, only 3 days after the release of the game itself, to correct one of the (at the time) most-complained problems of the game: having to switch between Flashlight and the weapons. The name refers to the "no duct tape in Mars" problem; as said on the mod's home page, "Under the crazy presumption that a roll of duct tape has to exist somewhere on the Mars facility, the Duct Tape mod sticks flashlights to your machinegun[sic] and shotgun."
To balance things out, only the shotgun and machine gun have duct-taped lights, and the beam is narrower than the regular flashlight's. Weapon models aren't modified to reflect the duct-taping.
According to the author, the mod was downloaded over 80,000 times from the official home page alone over the first 24 hours after the release.
The last update to the mod (version 0006) was in September 8, 2005, to make it compatible with Doom 3 v1.3. The mod is compatible with both the regular game and Resurrection of Evil; it can be installed and uninstalled without needing to start the game over.
As the game's inability to use the flashlight with weapons was caused by a technical limitation, the BFG Edition of Doom 3, released in 2012, featured an "armour-mounted" flashlight that could be toggled on and off at any time, regardless of what weapon is selected, essentially a superior official version of the Duct Tape mod.